# What's the reason that we have a different number of days each month?

It always was interesting for me to understand the answer for the question:

What's the reason that we have a different number of days each month?

If the month is fixed on the time that the moon circles the earth, then the time of the circling isn't equal each month? And if the month fixed by the time that the earth circles the sun and then divided by 12 (365:12=30.4), why one month has more days and the other month has less days?

Before desktop computers, how did astronomers handle calculations of dates and times accurately, for example two plates taken decades apart, how would an astronomer calculate the time difference precisely?

You make a great point. The reason behind the discrepancy between the dates is due to a complicated history behind it.

The calendar is based on the calendar created by ancient Romans, which is based on one Moon cycle. One lunar cycle is

29.53 days.

www.universetoday.com/20620/lunar-year/

which does not evenly divide into the 365.25 days of the year.

Note: The Romans borrowed the calendar from the ancient Greeks to develop the idea of a 10-month calendar that left approximately 60 days unaccounted for.

Consequently, the earliest ancient Roman calendars had months that were either 29 or 30 days to account for this discrepancy.

The Romans started using the 10-month calendar in 738 B.C. Their months were initially called:

• Mensis Martius (March)
• Mensis Aprilis (April)
• Mensis Maius (May)
• Mensis Iunius (June)
• Mensis Quintilis (July, quin- meaning "five")
• Mensis Sextilis (August, sex- meaning "six")
• Mensis September ("seven")
• Mensis October ("eight")
• Mensis November ("nine")
• Mensis December ("ten")

To account for the remaining $$sim60$$ days, Mensis Ianuarius (January) was added to the beginning of the year and Mensis Februarius (February) to the end of the year during Numa's reign around 700 B.C. with a leap year every 4 years.

Leap years were decreed by proclamation the pontifex maximus, in such years an extra month was added to the calendar.

Quoted from PM 2Ring

The calendar stayed in that order until 452 B.C. when a small council of Romans, called the Decemvirs, moved February to follow January.

However,

in the initial 36 years of its adoption leap days were added every 3 years due to a misunderstanding (Romans used inclusive counting),

Quoted from PM 2Ring

so that caused a big problem. In fact, by the start of the reign of Julius Caesar, the previous calendar was off by a whole week!

Julius Caesar noticed this and thus modified the Roman calendar in 46 B.C. to make each month have either 30 or 31 days, with the exception of Februarius[February], which had 29 days, with the three year leap year fixed into a four year leap year. Quintilis[July] was later renamed Julius[July] in his honor. Likewise, Sextilis[August] later became Augustus[August] to honor Augustus (Gaius Octavius, the first Roman Emperor). Augustus[August] was also given an extra day (taken away from Februarius[February]), so that Augustus and Julius would have an equal number of days, representing their equal power. This is the Julian Calendar.

This calendar was extremely accurate, but was still not accurate enough as it drifts by approximately 3 days every 400 years. So after a few hundred years, the drift becomes noticeable (i.e. the seasons don't properly match the calendar), which is important for farming, and therefore human survival.

quoted from CJ Dennis

Then a pope named Pope Gregory XIII modified the Julian calendar again in October 1582. This is the currently used Gregorian calendar. He made it so that

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400.

https://en.wikipedia.org/wiki/Gregorian_calendar

This calendar is the most accurate calendar currently used, and won't be a single day off until the year of $$pm 3200$$ AD. It isn't perfect though. Compared to the tropical year, it is 27 seconds too long. This is…

due to astronomical effects (the rotation of the Earth speeding up and slowing down, etc.) we can't be more precise than that.

quoted from CJ Dennis

As for your other question, unless the plates taken had accurate dates of when recorded, it would be rather impossible to calculate precisely.

## Why Does a Week Have 7 Days?

It's Monday and you are looking forward to the weekend. Why do you have to wait 5 days until Saturday? And how come the day after Monday is Tuesday and not, say, Sunday? You have an ancient people in modern-day Iraq to thank.

The lion of Babylon statue in Iraq.

## What's the reason that we have a different number of days each month? - Astronomy

December 2003 answer: It appears that the Egyptians were responsible for the 24 hour day. The Eqyptians were fond of counting in base twelve (instead of base 10 which is commonly used today). This is thought to be because they counted finger joints instead of fingers. Each of your fingers has three joints, so if you count by pointing to finger joints with your thumb you can count to twelve on each hand. This might seem arbitrary, but is actually just a strange as counting in base ten simply because we have ten digits.

(Feb 2004 Update: Thanks to a "Curious" reader for pointing out that another reason the Egyptians (and Indians) liked counting in base 12 is that 12 has a larger number of integer factors than 10. ie. 12/6=2, 12/4=3, 12/3=4, 12/2=6, while 10/5=2 and 10/2=5 are all there are for the number 10).

The Egyptians divided the clock into 12 hours of daytime and 12 hours of night-time (or alternatively 10 hours between sunrise and sunset, an hour for each twilight period and 12 hours of darkness). This is known because of various sundials from the period which have been found to be marked with hours. Interestingly this means that hours started out changing in length with the seasons (as the amount of daylight vs. darkness changes).

There is a more in-depth explanation for the division of night-time into 12 hours which is based on the number of "decan" stars which were seen to rise during summer nights in Ancient Egypt. A "decan" star was a star which rose just before sunrise at the beginning of a 10-day "decade" in Ancient Egypt. 36 "decan" stars marked the passage of a year for the Egyptians (or 36 10 day periods). During summer nights, 12 decan stars rose - one for each "hour".

However, hours did not have a fixed length until the Greeks decided they needed such a system for theoretical calculations. Hipparchus proposed dividing the day equally into 24 hours which came to be known as equinoctial hours (because they are based on 12 hours of daylight and 12 hours of darkness on the days of the Equinoxes). Ordinary people continued to use the seasonally varying hours for a long time. Only with the advent of mechanical clocks in Europe in the 14th Century, did the system we use today become common place.

Follow-up question (Apr 2006): How come there are 36 decan stars but only 12 in a night. Why aren't there 18 each night? Are some of the decan stars below the southern horizon part of the year. I don't understand how 36 of them equal 24 hours it seems to me they'd equal 24/36 = 2/3 hour each. What am I missing here?

There was not a decan star every modern hour. Remember that the length of darkness in the summer is actually less than 12 "modern" hours. The Egyptian "hours" marked by the rising of each of the 12 decan stars were shorter than what we call an hour now. As I said, hours did not have a fixed length until much later when people decided that would be useful! Initially 12 hours was always the length of night/day, but the hours themselves changed in length with the seasons, and a nighttime hour would have been different to a daytime hour! The "hours" in this era were only equal to our current hours on the equinoxes.

Is there a list of the decan stars somewhere?

I could not find a list of decan stars (or star groups in some cases), in modern terminology. A list of them in Egyptian terms is here.

#### Karen Masters

Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.

## Why Are There Seven Days in a Week?

Waiting for the weekend can often seem unbearable, a whole six days between Saturdays. Having seven days in a week has been the case for a very long time, and so people don’t often stop to ask why.

Most of our time reckoning is due to the movements of the planets, Moon and stars. Our day is equal to one full rotation of the Earth around its axis. Our year is a revolution of the Earth around the Sun, which takes 365 and ¼ days, which is why we add an extra day in February every four years, for a leap year.

But the week and the month are a bit trickier. The phases of the Moon do not exactly coincide with the solar calendar. The Moon cycle is 27 days and seven hours long, and there are 13 phases of the Moon in each solar year.

Some of the earliest civilizations observed the cosmos and recorded the movements of planets, the Sun and Moon. The Babylonians , who lived in modern-day Iraq, were astute observers and interpreters of the heavens, and it is largely thanks to them that our weeks are seven days long.

The reason they adopted the number seven was that they observed seven celestial bodies — the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. So, that number held particular significance to them.

Other civilizations chose other numbers — like the Egyptians, whose week was 10 days long or the Romans, whose week lasted eight.

The Babylonians divided their lunar months into seven-day weeks, with the final day of the week holding particular religious significance. The 28-day month, or a complete cycle of the Moon, is a bit too large a period of time to manage effectively, and so the Babylonians divided their months into four equal parts of seven.

The number seven is not especially well-suited to coincide with the solar year, or even the months, so it did create a few inconsistencies.

However, the Babylonians were such a dominant culture in the Near East, especially in the sixth and seventh centuries B.C., that this, and many of their other notions of time — such as a 60-minute hour — persisted.

The seven-day week spread throughout the Near East. It was adopted by the Jews, who had been captives of the Babylonians at the height of that civilization’s power. Other cultures in the surrounding areas got on board with the seven-day week, including the Persian empire and the Greeks.

Centuries later, when Alexander the Great began to spread Greek culture throughout the Near East as far as India, the concept of the seven-day week spread as well. Scholars think that perhaps India later introduced the seven-day week to China.

Finally, once the Romans began to conquer the territory influenced by Alexander the Great, they too eventually shifted to the seven-day week. It was Emperor Constantine who decreed that the seven-day week was the official Roman week and made Sunday a public holiday in A.D. 321.

The weekend was not adopted until modern times in the 20th century. Although there have been some recent attempts to change the seven-day week , it has been around for so long that it seems like it is here to stay.

## 3. Background of the Jewish calendar.

In the Old Testament, written in the Book of Exodus, that the Jewish have used a lunisolar calendar ever since they departed Egypt. It’s believed that “the first commandment the Jewish people received as a nation was to determine the New Moon,” notes Hebrew Calendar Facts for Kids. “Very soon after that, the Jews received the commandment to make sure that Passover falls in the spring.”

Both the Israelites and the Babylonians heavily influenced the creation of the calendar. But, it was between 70 and 1178 BCE that the ancient observation-based calendar was replaced with one that was more calculated by mathematics. The month names of the modern Hebrew calendar were derived from the Babylonian calendar dating back to the 6th Century BC during the Babylonian exile.

The Jewish calendar was mainly used to establish dates for holidays were the corresponding portions of the Torah were read to the public. But, following the destruction of the Second Temple in 70 CE, it wasn’t as easy for people to meet and listen to these passages. As a result, “the Sanhedrin established a fixed, rule-based form of the calendar. Maimonides fully described the modern Hebrew calendar around the year 1178 CE.

However, Persian astronomer al-Khwarizmi (c. 780 – 850 CE), claimed that the features of the modern-day version were created in the 9th century CE.

Also, in the modern Hebrew calendar, years are counted as Anno Mundi. Anno Mundi is Latin for “year of the world” and represents the traditional count of years since the creation of the world — as described in Genesis.

## Why the Week Has Seven Days

No more logic supports seven days than, say eight or five, yet the seven-day week has resisted any and all attempt at change.

Unlike the year and the month, which correspond to the movements of the earth around the sun and the moon around the earth, the week has no astronomical analogue. Seven days is roughly the same length of a phase of the moon (seven days, nine hours) but any system of timekeeping quickly falls apart due to the extra hours piling up.

Officially adopted by the Emperor Constantine in A.D. 321, the seven-day week goes back thousands of years. Here’s some history…

• The seven-day week started with the first civilizations of the Middle East. Mesopotamian astrologers designated one day for each of the seven most prominent objects in the sky—the Sun, the Moon, and the five major planets visible to the naked eye.
• The Jews also adopted a seven-day cycle, based on the time it took the Lord to create the universe as reported in Genesis. A new wrinkle in their week was the Sabbath, a day set aside for rest. This was the first time a culture had invented a holiday that occurred on a regular basis, unrelated to natural phenomena.
• The Romans adopted different weeks for business reasons they thought of a week as the 8 days between market days.
• West African societies preferred a four-day market cycle.
• In Assyria, 6 days was the rule in Egypt, 10 in China 15.
• The ancient Germans used a five-day cycle named for their primary gods which is how our week ended up honoring Norse deities like Tiw (Tuesday), Odin (Wednesday), Thor (Thursday), and Frigga (Friday). Our word “week” may come from the Old Norse word vikja, which means “to turn.” Sunday and Monday, or course, honor the Sun and the Moon.

Atheistic revolutionaries tried, unsuccessfully, to get rid of the seven-day week. In 1793 the leaders of the French Revolution produced a new calendar divided into three ten-day “decades.” It never caught on, and Napoleon abandoned it in 1805.

In 1929 the Soviet Union tried a five-day week, with one day of rest.

• Instead of the traditional day names, the days were given colors: yellow, orange, red, purple, and green.
• In order to keep mass production going, each Soviet citizen was assigned a different day of rest, so that a husband might have a yellow day off, while his wife took her leisure on green.
• Due to mass confusion, the plan was revised in 1932 to a six-day week, with numbers replacing the colors.
• By 1940, the Russians were back on the familiar seven-day cycle!

Somebody was always trying to come up with something better! In 1936 the League of Nations solicited proposals for world calendar reform and considered almost 200 different schemes, many of which rearranged the week.

• Edward Skille of Drummund, Wisconsin, suggested a year consisting of 73 five-day weeks called “metos.” The days of the week would be called Ano, Beno, Ceno, Deno, and Eno.

Why the seven-day week? It doesn’t divide evenly into 365- or 366-day years, so that holidays fall on different days of the week from year to year. But human beings are not logical creatures. Who can imagine saying, “What are you doing next purple?” or “Yeah, it’s Eno!”

As one member of the British Parliament remarked in a 1944 debate on calendar reform, “It is bad enough to be born on April 1, but to have one’s birthday always on a Monday would be perfectly intolerable.”

## How to Memorize How Many Days Are in Each Month

This article was co-authored by our trained team of editors and researchers who validated it for accuracy and comprehensiveness. wikiHow's Content Management Team carefully monitors the work from our editorial staff to ensure that each article is backed by trusted research and meets our high quality standards.

wikiHow marks an article as reader-approved once it receives enough positive feedback. In this case, 81% of readers who voted found the article helpful, earning it our reader-approved status.

Do you ever need to remember the number of days in a month offhand? You may not always have a calendar handy when you need one. It may be easier to have the number of days in each month memorized. There are a few simple tricks you can use to remember the number of days in each month. There's a common poem people use to remember, and you can also use a trick involving the knuckles of your hands. If this fails, you can use common tricks to improve your memory to help solidify the information.

## Related Articles

### Group seeking crowdfunding to put a Torah on the moon

This reckoning was instituted by Maimonides in the 12th century, in the stead of the previous system Jews had used before, which counted from the destruction of the Temple in 70 CE.

Moving onto today: Israel's official calendar is the Hebrew one. Under law, official Israeli documents must have the Hebrew date on them. Moreover, holidays in Israel are determined according to the Jewish calendar, not the Gregorian one. Thus a given fest – say, Rosh Hashanah – will happen on the same date each year according to Jewish reckoning, but on a different day each year according to the Gregorian calendar. That is because the Gregorian and Jewish calendars don't coincide.

Even civil holidays in Israel, such as Jerusalem Day, are based on the Jewish calendar.

Yet in their day to day lives, most Israelis are completely unaware of the Hebrew date and lead their lives according to the Gregorian calendar. That said, a not-insignificant religious minority still adhere to the Hebrew calendar of old.

The Hebrew calendar is very complicated, because it has to align the solar year (365 days, 5 hours, 48 minutes and 46-seconds) with the lunar year (12 months of 29 days, 12 hours, 44 minutes and 3 seconds).

This means that it must make allowance for the fact that 12 lunar months are about 12 days shorter than the solar year. This isn’t easy.

If you get it wrong, even by a little bit, over time the two would slowly drift apart and you’d have your spring holidays in the fall.

An ancient limestone calendar

Like most ancient peoples, at first the Jews followed a strictly lunar calendar. Our earliest record of this is a 10th century BCE calendar found in the Canaanite town of Gezer (midway between Jerusalem and Tel Aviv).

#### Thank you for signing up.

We've got more newsletters we think you'll find interesting.

#### Thank you,

Regarding the "Israelite new year," this seems to have swung back and forth over the centuries. The Gezer calendar seems to place the "start" of the new year in the fall by the time the Torah was redacted some four centuries later, the new year had moved to the spring. Today it's celebrated in the fall again. (It perhaps bears mention that the ancient Hebrews saw no need that we know of to mark the passing of an old year and start of a new one.)

Maybe calling the inscribed limestone tablet found at Gezer a "calendar" is an exaggeration. The etched stone, in either Phoenician or paleo-Hebrew (depending on who you ask) lists months (and month duos), which don't have proper names, just designated times of the year.

These months are referred to as “moon(s) of X” with X being the main agricultural activity of that month (or two months).

For example, the list begins in the fall with yarkhu asif, that is, “two moons of harvesting.”

As we said, some four centuries later when the Torah was being written, the year was considered to begin in the spring, with each month named by its ordinal number. Thus the spring month on which Passover fell was called the “first month,” the next month was the “second month” and so on.

The arrival of spring

The solar calendar is automatically in line with the seasons, but being some 12 days shorter, the lunar calendar is not.

The Bible doesn’t tell us how the lunar calendar was periodically adjusted to keep in line with the seasons. But it must have been, at least from the days of King Josiah and the centralization of the cult in Jerusalem (in the second half of the seventh century BCE). On Passover, Shavuot, and Sukkot – which have specific Hebrew dates - the people of Israel were required to bring agricultural produce to Jerusalem and if there had been "seasonal creep," in at least some years, there would have been no produce to bring as tithes.

It is almost certain this is how they adjusted for seasonal creep: At the end of the 12th lunar month, priests in Jerusalem determined spring had arrived. If spring was indeed deemed to have arrived, great: the new moon was a sign that not only the first month had begun but also a new year, and messengers were sent out to announce that Passover was two weeks away.

But if the priests decided that it was still winter, an additional “leap month” or “embolismic month” as its called was added, and messengers would only be sent out a month later.

Thus the calendar was kept in line with the seasons. It is possible that this process was made more precise with the introduction of a sundial of sorts introduced by King Ahaz in the end of the eighth century BCE.

Jewish month - named for Babylonian god

It seems that before their banishment from the Holy Land and exile in Babylon, the ancient Hebrews hadn't given their months actual names. But after the Babylonian exile in the Second Temple period, the months began to be called by their Akkadian Babylonian names - which we use to this very day.

Some of these names are innocuous enough, such as Tishrei, meaning “beginning,” Marheshvan was shortened to Heshvan meaning “eighth month,” and so on. Other month names though are quite surprising: Tammuz for example originated with a pagan Babylonian god.

Another change that happened gradually in the Second Temple period was that the year was increasingly seen as beginning in the fall, as it is to this very day, and not in the spring as it was in biblical times.

The hand of imperfect men

At first the decision on when exactly one month ended and another began, and when a leap month was to be added, was in the hand of men. But in a gradual process spanning hundreds of years, a fixed system was devised, which we use to this day.

According to the Mishnah (redacted 200 CE), during the time of the Second Temple, witnesses would come to the Temple and report that they had seen the new moon. They would be grilled by priests, who would decide whether a new month had in fact begun.

If they decided it had, the priests would announce the new moon to the public.

Thus they would decide the length of each month and whether an embolismic month was to be added to the year. When the Temple was destroyed by Titus in 70 CE, these functions were taken up by the rabbinic council, the Great Sanhedrin, and more specifically by a subcommittee of three of the Sanhedrin members that would meet on the 29th of every month and receive witnesses and come to decisions regarding the calendar.

From Mishnaic times through to the ninth century CE at the latest, the Hebrew calendar gradually lost the errant human element and shifted to a fixed calendar based on predetermined rules and calculations.

Samuel the astronomer

A key figure in the process of formalizing the calendar from observation to calculation was the great Babylonian rabbi Samuel of Nehardea, who was a great astronomer. The Talmud quotes him as saying “I am as familiar with the courses of the stars as with the streets of Nehardea” (Berachot 58b).

Samuel developed the calculations necessary to determine the calendar without relying on messengers from Palestine, which, he explained, meant that Jews outside of Palestine no longer had to add an extra day to their holidays, though halakha still prescribes that these extra days should be kept just the same.

Throughout the leadership of Judah III (290-320 CE), the Great Sanhedrin continued to receive witnesses of the new moon, but only as a formality, and for the sake of preserving time-honored tradition. The decision was already made according to calculation.

Samuel's son and successor as Nasi of the Sanhedrin, Hillel II (330-365 CE) is credited with finalizing the fixed Hebrew calendar as we know it, though the rules we know today were only fully laid out in writing in the 9th century.

Morphing months

So how does it work? The basic Jewish year has 12 months with five months of 29 days, and five months of 30 days, which alternate. The two other months - Heshvan and Kislev - change from year to year, according to the rules elaborated below.

But not all Jewish years have 12 months. Seven out of every 19 years are "embolismic years," which means they have a 13th month, called "Adar Bet" or - a "second Adar".

Whether a year is embolismic or not is determined by its place on a 19-year cycle. Most years are “regular” 12-month years except for years 3, 6, 8, 11, 14, 17, and 19, which are embolismic.

Each year, whether embolismic or not, starts on Rosh Hashanah - the day the new moon of Tishri appears. But if one of four conditions called dekhiot (“postponements”) are met, the Rosh Hashanah holiday - and thus the beginning of the year - is postponed by a day or two. In practice, that happens in most years.

So what are these postponing rules? (1) If Rosh Hashanah falls on Sunday, Wednesday or Friday, it is postponed by a day. (2) If the new moon appears after noon, Rosh Hashanah is postponed by a day. If the new day is one of those in Rule 1, Rosh Hashanah gets postponed by two days. (3) In a regular 12-month year, that is one that isn’t embolismic, if the new moon of Tishri appears more than 20 seconds past 3:11 A.M. on a Tuesday, Rosh Hashanah is postponed by two days.

The last instance is an extremely rare one. (4) In years that follow an embolismic year, if the new moon appears on a Monday more than 43 seconds past 9:32 A.M. - then Rosh Hashanah is postponed by a day.

You might assume these one- or two-day postponements simply cause one or two days to be added to the last month of the year, Elul. Not so! In fact they were added six months in advance to the winter months of Heshvan and Kislev, which as we said, change in length from year to year.

Theoretically Heshvan and Kislev each have 29 days. But if the new year needs to be postponed by one day, the preceding Heshvan is expanded to 30 days and Kislev stays at 29.

If two days are to be added, then both the preceding Heshvan and Kislev have 30 days.

How is this calculated six months in advance? Because dekhiot can be and are calculated years in advance using moon charts and math. When calculating the calendar for the year to come, you can calculate what kind of year it will be well in advance, and monkey with the lengths of Heshvan and Kislev as needed.
That’s it. If you have the data on when the new moon will rise over Jerusalem for each year, information that is readily available online, you can calculate the Hebrew calendar yourself. Though, thankfully, you don’t have to. Shanah Tova!

## Contents

Only a few stars and constellations are named individually in the Hebrew Bible, and their identification is not certain. The clearest references include:

• Kesîl (כְּסִ֥יל), [2] usually understood to be Orion, a giant angel.
• Kîmāh (כִימָ֗ה), [3] which may be the Pleiades, Aldebaran, Arcturus, or Sirius.
• 'Ash or 'Ayish (עָ֭שׁ ‘Āš), [4] possibly the Hyades, Arcturus or Ursa Major, or even the Evening Star (Venus when seen at dusk).
• Məzārîm (מזרים), [5] which may be Ursa Major and Ursa Minor, or a synonym for mazzalot, in which case it would refer to the planets or the constellations of the zodiac.

Aside from the Earth, only two planets are named in the Hebrew Bible:

• The term כיון (Kîyūn, Chiun) in Amos 5:26[6] is thought by some authors to refer to Saturn, closely related to the Assyrian "Kévan" or "kaiwanu." [7]
• Venus, called meleḵeṯ ha-šāmayim, (מְלֶ֣כֶת הַשָּׁמַ֗יִם), "the queen of heaven," in Jeremiah 7:18 and elsewhere. That the latter means Venus is shown by the cakes which are said to have been baked for her. [8] Among the Assyrians and Babylonians, the cake offerings were called "the bread of Ishtar."
• Hêlêl (הֵילֵ֣ל), the "son of the morning," (בֶּן שָׁ֑חַר Ben-Šāḥar) in Isaiah 14:12, is also thought to be the morning star (Venus when visible before dawn). This identification is better known to many English speakers as its Latin name, Lucifer, the "light-bearer."

The information preserved in the Talmud does not emanate from one homogeneous system, as they are the accumulations of at least four centuries, and are traceable to various authors in the Jerusalem and Babylonian Talmuds, among whom some were inclined to mysticism.

### Astronomy as a religious study Edit

The high value of astronomical knowledge is already demonstrated by the astronomical section of the Book of Enoch (about 72-80 BC), as well as by such sayings as those of Eleazar Hisma (about 100), a profound mathematician, who could "count the drops in the ocean", [9] and who declared that the "ability to compute the solstice and the calendar is the 'dessert' [auxiliaries] of wisdom. [10] Among the sciences that Johanan ben Zakkai mastered was a knowledge of the solstices and the calendar i.e., the ability to compute the course of the Sun and the Moon. [11] Later writers declare that "to him who can compute the course of the sun and the revolution of the planets and neglects to do so, may be applied the words of the prophet, [12] 'They regard not the work of the Lord, nor consider the operation of His hands.'" To pay attention to the course of the sun and to the revolution of the planets is a religious injunction for such is the import of the words, [13] "This is your wisdom and your understanding in the sight of the nations". [14]

Despite the general importance and religious significance attached to astronomy in the Holy Land, no notable developments in astronomy happened there. The starry heavens of the land of Israel interested the Jews as creations of God and as means to determine the holidays, but for a better knowledge of them the Jews were undoubtedly indebted to the Babylonians and their Hellenic pupils, as evidenced by the foreign term gematria used to designate the computation of the calendar. Probably this word represents a transposition of the Greek γραμματεία meaning "arithmetic, mathematics." Most of the observations of a scientific nature were transmitted by Samuel of Nehardea, who attended the schools of the Babylonians, and who claimed to possess as exact a knowledge of the heavenly regions as of the streets of Nehardea. Certain rules must nevertheless have existed, because Rabban Gamaliel (about 100), who applied the lunar tablets and telescope, relied for authority upon such as had been transmitted by his paternal ancestors. [15]

### Correspondence between Biblical numbers and astronomy Edit

The number 7 is a recurring numerical theme in the Hebrew scriptures. The menorah's seven lamps on four branches correspond to the lights of the seven Classical planets: the Moon, Mercury, Venus, the Sun (4th), Mars, Jupiter, and Saturn. Hebrew mysticism recognized their great importance. Therefore, along with the four lunar phases being slightly over seven days (

7.4 days) each, the number 7 was held in very high regard. The Torah reflects this with Bereishit (Book of Genesis 1:1) being seven words and twenty-eight letters (7x4) in its original Hebrew. This is "God's signature". [ citation needed ]

"And God said, 'Let there be lights in the heavens to separate the day from the night, and let them serve as signs to mark seasons, days, years and festivals'. the 4th day (of 7)." [16]

Some scholars identified the 12 signs of the zodiac with the 12 sons of Jacob/twelve tribes of Israel. [17]

### Conceptions of the Heavens and Earth Edit

In the Talmud, as in the Bible, the heavens and the Earth designate the two borders of the universe, with the heavens a covering over the Earth. One tannaitic authority believed that the sphere consists of a strong and firm plate two or three fingers in thickness, always lustrous and never tarnishing, another estimates the diameter of this plate as one-sixth of the Sun's diurnal journey, [ citation needed ] while another, a Babylonian, estimates it at 1,000 parasangs (approx. 3728 miles). Yet another authority states that the diameter of the firmament is equal to the distance covered in 50 or 500 years and this is true also of the Earth and the large sea (Tehom) upon which it rests. [18]

The distance of the firmament from the Earth is a journey of 500 years, a distance equivalent to the diameter of the firmament, through which the Sun must saw its way in order to become visible. [19] The firmament, according to some, consists of fire and water, and, according to others, of water only, while the stars consist of fire. [20] East and West are at least as far removed from each other as is the firmament from the Earth. [21] Heaven and Earth "kiss each other" at the horizon and between the water above and that below there are but two or three fingerbreadths. [22] The Earth rests upon water and is encompassed by it.

According to other conceptions the Earth is supported by one, seven, or twelve pillars. These rest upon water, the water upon mountains, the mountains upon the wind, and the wind upon the storm, though this could easily be metaphoric. [23] There is also mentioned the terrestrial globe, kaddur.

### Chronology and the Mazzaroth Edit

Chronology was a chief consideration in the study of astronomy among the Jews sacred time was based upon the cycles of the Sun and the Moon. The Talmud identified the twelve constellations of the Mazzaroth (zodiac) with the twelve months of the Hebrew calendar. [ citation needed ] The correspondence of the constellations with their names in Hebrew and the months is as follows:

- Ṭ'leh - Nisan - Shor - Iyar - Teomim - Sivan - Sarṭan - Tammuz - Ari - Av - Betulah - Elul - Moznayim - Tishrei - 'Aḳrab - Cheshvan - Ḳeshet - Kislev - Gedi - Tevet - D'li - Shevat - Dagim - Adar

The first three are in the east, the second three in the south, the third three in the west, and the last three in the north and all are attendant on the Sun. According to one account, in the first three months (spring) the Sun travels in the south, in order to melt the snow in the fourth through sixth months (summer) it travels directly above the Earth, in order to ripen the fruit in the seventh through ninth months (autumn) it travels above the sea, in order to absorb the waters and in the last three months (winter) it travels over the desert, in order that the grain may not dry up and wither.

According to one conception, Aries, Leo, and Sagittarius face northward Taurus, Virgo, and Capricornus westward Gemini, Libra, and Aquarius southward and Cancer, Scorpio, and Pisces eastward. Some scholars identified the twelve signs of the zodiac with the twelve tribes of Israel.

The four solstices (the Tekufot of Nisan, Tammuz, Tishrei, and Tevet) are often mentioned as determining the seasons of the year and there are occasional references to the rising-place of the Sun. [24] Sometimes six seasons of the year are mentioned, [25] and reference is often made to the receptacle of the Sun (ναρθήκιον), by means of which the heat of the orb is mitigated. [26] The Moon was also a part of the calendar: "The moon begins to shine on the 1st of the month its light increases until the 15th, when the disk [דסקוס (δίσκοσ)] is full from the 15th to the 30th it wanes and on the 30th it is invisible." [27]

### The heavenly bodies and their motions Edit

Two different cosmologies can be found in the Talmud. One is a flat Earth cosmology resembling descriptions of the world in the mythology of the Ancient Near East. The other, is a geocentric model, according to which the stars move about the earth. According to Aristotle, Ptolemy, and other philosophers among the Greeks, the stars have no motion of their own, being firmly attached to spheres whose center is the Earth. A passage in the Talmud contrasts the pagan view with that of Jewish sages:

The learned of Israel say, "The sphere stands firm, and the planets revolve" the learned of the nations say, "The sphere moves, and the planets stand firm." The learned of Israel say, "The sun moves by day beneath the firmament, and by night above the firmament" the learned of the nations say, "The sun moves by day beneath the firmament, and by night beneath the earth." [28]

The Sun has 365 windows through which it emerges 182 in the east, 182 in the west, and 1 in the middle, the place of its first entrance. The course described by it in a year is traversed by the moon in 30 days. The solar year is longer by 11 days than the lunar year. [29] The Sun completes its course in 12 months Jupiter, in 12 years Saturn, in 30 years Venus and Mars, in 480 years [30] however, an objection is raised here (in a gloss) against the last-mentioned number. King Antoninus asked the patriarch why the Sun rises in the east and sets in the west. At the time of the Deluge it traveled in the opposite direction. [31] Every 28 years it returns to its original point of departure, and on Tuesday evening of the spring solstice it is in opposition with Saturn, although Plato maintained that the Sun and planets never return to the place whence they started. This is the cycle of 28 years. [32] The Moon-cycle of 19 years may have been meant in Targum Pseudo-Jonathan Genesis 1:14.

The names of the five planets, one star and one moon [planetary satellite] are: [33]

• Shabbatai שבתאי, Saturn
• Meaning: "the restful one," whose name is parallel to that of the Sabbath Day the Seventh day the day of rest. Esoterically, Saturn embodies Time itself. In the midst of time's passage, Saturn remains still and silent, drawing all endeavors to a close.
• Meaning: "righteousness", as Jupiter is the embodiment of divine influx.
• Meaning: "the red one"
• Meaning: "the hot one"
• Meanings: "the she-planet," "the bright one," or "the bright planet," respectively
• Meaning: "the planet," since Mercury is the Principle of planetary influence, in and of itself. The Mercurial principle is that of multiplorancy. It embodies our means of adaptation, and represents the many facets of existence. In being deemed simply as "the planet", Mercury is presented as a blank slate an open-ended modality of being.
• Meaning: "the white one"

In many languages, the names of the days of the week are derived from the names of the seven planets each day was consecrated to the particular planet that ruled during the early hours of the morning. While Talmudists were familiar with the planets and their characteristics in astrology, they opposed their worship, so weekdays are not named in Hebrew besides for the Sabbath. Instead they are referred to by number.

### Fixed stars and comets Edit

The Milky Way is called "Fire-Stream," a name borrowed from Daniel 7:10 (Nehar di-nur), where it may possibly have had the same signification. The statement is also made that the sting of Scorpio may be seen lying in the Milky Way. [34] Samuel said: "We have it as a tradition that no comet ever passed across the face of Orion "Kesil" for if this should happen the earth would be destroyed." When his hearers objected to this statement, saying, "Yet we see that this occurs," Samuel replied: "It only appears so for the comet passes either above or below the star. Possibly also its radiance passes, but not its body." Again, Samuel says: "But for the warmth of Orion, the earth could not exist, because of the frigidity of Scorpio furthermore, Orion lies near Taurus, with which the warm season begins. [35] The comet, because of its tail, is called kokba de-shabbiṭ. (rodstar). Rabbi Joshua ben Hananiah (about 100), declared that a star appears once every seventy years and leads mariners astray, hence they should at such time lay in a larger store of provisions. [36] Rapoport endeavors to prove that the path of Halley's comet had been computed by a wise rabbi. [37] Samuel said: "I know all the paths of heaven, but nothing of the nature of the comet."

The following Biblical names of constellations are mentioned and explained: כימה = כמאה. Pleiades (Biblically known as the Seven Stars) [a cluster of] about a hundred stars, and for the much disputed עש, its equally obscure Aramaic equivalent יותא (MS. M. אתא), Syriac עיותא, is given. [38]

With the revival of Hellenistic astronomy which took place during the Islamic Golden Age, Jews were intimately connected, and the Almagest is said to have been translated by Sahal ibn Tabari as early as AD 800, while one of the earliest independent students of astronomy among the Arabs was Mashallah ibn Athari (754-873?). Jews seem to have been particularly concerned with the formation of astronomical tables of practical utility to astronomers. Sind ben Ali (about 830) was one of the principal contributors to the tables drawn up under the patronage of the Caliph al-Mamun. No less than twelve Jews were concerned in the Tables of Toledo, drawn up about AD 1080 under the influence of Ahmad ibn Zaid, and the celebrated Alfonsine Tables were executed under the superintendence of Isaac ibn Sid, while Jews were equally concerned in the less-known tables of Peter IV of Aragon.

Isaac al-Ḥadib compiled astronomical tables from those of Al-Rakkam, Al-Battam, and Ibn al-Kammad. Joseph ibn Wakkar (1357) drew up tables of the period 720 (Heg.) while Mordecai Comtino and Mattathia Delacrut commented upon the Persian and Paris tables respectively the latter were commented upon also by Farissol Botarel. Abraham ibn Ezra translated Al-Mattani's Canons of the Khwarizmi Tables, and in his introduction tells a remarkable story of a Jew in India who helped Jacob ben Tarik to translate the Indian astronomical tables according to the Indian cycle of 432,000 years. Other tables were compiled by Jacob ben Makir, Emanuel ben Jacob, Jacob ben David ben Yom-Ṭob Poel (1361), Solomon ben Elijah (from the Persian tables), and Abraham Zacuto of Salamanca (about 1515).

The earliest to treatise of astronomy in Hebrew on a systematic plan was Abraham bar Ḥiyya, who wrote at Marseilles, about AD 1134. Discussions on astronomical points, especially with regard to the spheres, and disputed points in calculating the calendar occur frequently in the works of Judah ha-Levi, Abraham ibn Ezra, and Maimonides, while a new system of astronomy is contained in the "Wars of the Lord" ("Milḥamot Adonai") of Levi ben Gershon.

Jews were especially involved as translators. Moses ibn Tibbon translated from the Arabic Jabir ben Aflah's acute criticisms of the Ptolemaic system, an anticipation of Copernicus, and thus brought them to the notice of Maimonides. Ibn al-Haitham's Arabic compendium of astronomy was a particular favorite of Jewish astronomers besides being translated into Spanish by Don Abraham Faquin, it was turned into Hebrew by Jacob ben Makir and Solomon ibn Pater Cohen and into Latin by Abraham de Balmes. Other translations from the Arabic were by Jacob Anatoli, Moses Galeno, and Kalonymus ben Kalonymus, bringing the Greco-Arabic astronomers to the notice of western Europe. Jacob Anatoli, for example, translated into Hebrew both the Almagest and Averroes' compendium of it, and this Hebrew version was itself translated into Latin by Jacob Christmann. Other translators from the Hebrew into Latin were Abraham de Balmes and Kalonymus ben David of Naples, while David Kalonymus ben Jacob, Ephraim Mizraḥi, and Solomon Abigdor translated from the Latin into Hebrew. The well-known family of translators, the Ibn Tibbons, may be especially mentioned. In practical astronomy Jewish work was even more effective. Jacob ben Makir (who is known also as Profiat Tibbon) appears to have been professor of Astronomy at Montpellier, about 1300, and to have invented a quadrant to serve as a substitute for the astrolabe. Levi ben Gershon was also the inventor of an astronomical instrument, and is often quoted with respect under the name of Leon de Bañolas. Bonet de Lattes also invented an astronomical ring. Abraham Zacuto ben Samuel was professor of Astronomy at Salamanca, and afterward astronomer-royal to Emmanuel of Portugal, who had previously been advised by a Jewish astronomer, Rabbi Joseph Vecinho, a pupil of Abraham Zacuto, as to the project put before him by Christopher Columbus, who, in carrying it out, made use of Zacuto's "Almanac" and "Tables."

With the Renaissance, Jewish work in astronomy lost in importance, as Europe could refer to the Greek astronomers without it. The chief name connected with the revival of astronomical studies on the Baltic is that of David Gans of Prague (d. 1613), who corresponded with Kepler and Tycho Brahe. He was acquainted with the Copernican system, but preferred that of Ptolemy, while as late as 1714 David Nieto of London still stood out against the Copernican system.

Other Jewish astronomers of note are H. Goldschmidt (1802–66), who discovered 14 asteroids. Wilhelm Beer (1797–1850), the brother of Meyer Beer, drew one of the most accurate maps of the moon of his time. Moritz Loewy (b. 1833) was director of the Paris Observatory, and the inventor of the coudé or elbow telescope, by which the stars may be observed without bending the neck back and without leaving the comfortable observatory.

## What's the reason that we have a different number of days each month? - Astronomy

Reasons for Seasons
Teacher's Background Information
(Reasons for Seasons Mini-Unit)

Watching the wave of spring sweep northward is at the heart of Journey North. All seasonal changes – temperature, plant growth and life cycles, animal migrations, and so on – are driven by shifts in:

• the amount of available sunlight (called day length or photoperiod)
• the intensity of sunlight (related to the angle at which it strikes the Earth)

During our summer, the Northern Hemisphere leans toward the sun in its revolution, there are more daylight hours, and the sun’s angle is more perpendicular to us than at other times of year. The longer days and more concentrated sunlight and results in more heating. (Shadows are shorter in the summer because the sun strikes Earth more directly.)

During winter, the Northern Hemisphere leans away from the sun, there are fewer daylight hours, and the sun hits us at an angle this makes it appear lower in the sky. There is less heating because the angled sun’s rays are “spread out” rather than direct. (Shadows are longer because of the lower angle of the sun.)

In equatorial regions, the length of days and the directness of sunlight don't change as much. The further you get from the equator, the more dramatic the seasonal changes.

During the spring and fall, the Earth leans neither toward nor away from the sun daylight and nighttime hours are more equal and temperatures are moderate. (The shadow of an object is similar during these seasons.)

Common Misconceptions
Many students (and adults) believe that the Earth is closer to the Sun in the summer and further away in the winter. (It’s actually somewhat closer to the sun in the winter, but the angled rays and short days don’t give us much heat.) Another misconception is that the earth orbits the sun in an elongated ellipse, which makes the earth’s distance from the sun dramatically different at different locations. The reality is that the earth’s orbit is nearly circular.

Solstice vs. Equinox

Solstice refers to the two times each year when the sun's strongest rays are furthest from the equator (north of it during our summer solstice and south during the winter). For the northern hemisphere, summer solstice occurs around June 21st we have the maximum number of daylight hours at that time. Winter solstice is around December 21st when we have the fewest daylight hours.

Equinox refers to the two times each year when the sun's strongest rays are directly hitting the equator. Everywhere on earth has 12 hours of daylight on the spring and fall equinoxes. In the northern hemisphere, spring equinox occurs around March 21st and autumnal equinox around September 21st.