[History of Astronomy]
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Day - Week - Month - Seasons - Year - Millennium
But there are actually two types of day. If you were to look at the Earth from above the north pole you would see that the time it takes to complete a rotation on its axis is 23 hours 56 minutes and 4 seconds. This is the rotation period relative to the stars, so it is called the Sidereal Day, from a Latin word for star. Because the Earth is moving around the Sun, it takes nearly four minutes more for the Earth to face the Sun exactly. So, the time from sunrise to sunrise (or noon to noon) is 24 hours (by definition). Since the Sun is more important to us than the stars, it is the Solar Day that is more important. Indeed, the Solar Day is referred to as the day.
However, the Sun is not a very good time keeper. Except on April 15, June 15, August 31 and December 24, the sundial says it's noon at a different time to actual noon. The difference can be as much as 17 minutes. This is caused by the Earth's motions around the Sun not being regular. An effect of this is to make the sunrise and sunset times asymmetrical to noon. For example, in London on 9 February, the Sun rises at 7:29 and sets at 5:01. This makes 4 hours 31 minutes from sunrise till noon but 5 hours 1 minute after noon! On 2 November the sunrise / sunset times are 6:55 and 4:31 respectively (5 hours 5 minutes before noon, 4 hours 31 minutes after noon).
The question how long is the day has lots of answers.
Well actually, no, it doesn't. Except on 21 March and 23 September. On 21 March, the Sun rises exactly in the East, climbs up towards the South and (in London) reaches an altitude of about 39 degrees above the southern horizon. It then sets due West. The day is exactly 12 hours long, as is the night. 21 March is the Vernal Equinox. Vernal means 'spring', while equinox is from Latin words meaning 'equal night'.
After this date, the Sun rises a little north of East, climbs higher in the sky at noon, spending more time in the daytime sky than below the horizon, and sets a little north of West. This process continues until 21 June. On this date, the Sun rises at its furthest north of East. It climbs up to an altitude of 61 degrees above the southern horizon at noon and sets well north of West. On 21 June, the daylight period is over 16 hours long (remember those long summer evenings?), with only 8 hours, or so, of night. This is the Summer Solstice. On this one date, the sunrise at Stonehenge is over a particular stone specially aligned with the Solstice.
After 21 June, the Sun begins to move south so that by 23 September there is another Equinox (the Autumnal Equinox). The Sun continues its southerly travel until 21 December. On this date, the Sun rises south of East, barely climbs to a mere 16 degrees above the horizon, and sets early in the South West. It is the shortest day (less than 8 hours of daylight - remember the dark mornings and evenings?). This is the Winter Solstice.
On the Equator, all days contain 12 hours of daylight and 12 hours of night. The further away from the Equator you go, the larger the difference between the longest and shortest days. In North Africa, daylight varies between 10 and 14 hours, in London it varies between 8 and 16 hours. In Iceland it varies between 0 and 24 hours (yes, there is no sunset on the longest day - the so called Midnight Sun, and no sunrise on the shortest day!).
The Sun does rise the East and sets in the West ... twice a year.
The Babylonians named each day after these objects. These names have come down to the modern world via equivalent gods. For example, in English we have the Sun's Day (Sunday), the Moon's Day (Monday), Tiw's Day (Tuesday - Tiw is the Norse version of the god of war, Mars: in Spanish Tuesday is martes), Woden's Day (Wednesday - from Woden, the Norse version of Mercury - Miercoles in Spanish), Thor's Day (Thursday - Thor is the Norse god of thunder and battle, similar to Jupiter - Jueves in Spanish - even in Hindi, Braspati is Jupiter and Braspativar is Thursday), Frigga's Day (from Frigga, the Norse Venus), Saturn's Day (Saturday).
The Week is an artificial period of time.
Around 580 BC when the Israelites were under Babylonian rule, they had to keep a seven day Week. Since their religion forbade them from worshipping the stars, they rewrote the Babylonian creation stories, replacing the many Babylonian gods with their single God. The creation was therefore said to have taken seven days.
Although the Western world uses a Solar Calendar, there are still times when the older Lunar Calendar comes into play. For example, Easter is the first Sunday after the first Full Moon after the Spring Equinox. This is why this festival falls on a different day each year.
If the Moon is followed against the starry background, it completes a revolution around the Earth in 27 days 7 hours 43 minutes and 11.5 seconds, on average. In fact, this period can vary by several hours because of the gravitational pull of the Sun and other planets. This is called the Sidereal Month.
As the Moon orbits the Earth, the part lit by the Sun that we see changes. This produces the Phases of the Moon. The period from Full Moon to Full Moon (or Half to Half, New to New, etc) is called the Synodic Month. It has a period of 29 days 12 hours 44 minutes and 3 seconds. The actual value can vary by as much as 13 hours from the above average. As you can see, the Synodic Month averages about 2 days more than the Sidereal Month. This is because, the Moon's Phases depend on the Sun and its position changes because the Earth is moving around it.
The question how long is the month?, has the answer what month do you mean?
The Sidereal Year is the length of time required for Earth to revolve around the Sun, relative to the stars. It is 365 days 6 hours 9 minutes and 10 seconds. It is the year of the stars. The position of the Sun in the Zodiac is tied up with this year. This, however, is not the year that the solar calendar used by the West is based on.
The Tropical Year is the period from the Vernal Equinox to the Vernal Equinox (or Solstice to Solstice). It is the year of the seasons (from the Greek word for weather). It is 20 minutes shorter than the Sidereal Year (365 days 5 hours 48 minutes and 46 seconds). This is the year that the West bases its calendar on. Because of this, spring starts (in the Northern Hemisphere) on 21 March. However, because the Sidereal Year is not being used, the dates given by astrologers for the Zodiac signs are no longer correct. The 20 minute difference between the two types of year adds up to 1 day over 70 years and 1 month over 2000 years.
2000 years ago, the northern Spring began on 21 March and the Sun entered Aries. Had the Sidereal Year been used, the Sun would still be entering Aries on 21 March but Spring would be starting in late April. But the West went for the seasons (agriculture depends on the seasons). Now 2000 years later, Spring still starts on 21 March but the Sun is no longer entering Aries, but Pisces. In fact, all astrological dates are out by a month. This effect is called the Precession of the Equinoxes. It is caused by the Earth wobbling like a top as it spins on its axis. One complete wobble takes 26,000 years. The Greeks discovered this phenomenon.
What sign are you?
The seasons are caused by the fact that the Earth goes around the Sun at an angle. The Earth's axis is not perpendicular to its orbit. It has a tilt of 23.5 degrees. It is like a top spinning at an angle. During the year, the Earth's axis points constantly in one direction in space. As the Earth revolves around the Sun, each pole is tilted at the Sun for six months. When the North Pole is tilted towards the Sun, there is more sunshine in the Northern Hemisphere giving us the longer days of Spring and Summer. When the South Pole is tilted towards the Sun, it shines less in the Northern Hemisphere giving us shorter days and less Sun. If you look at globes of the Earth, they are never upright; they are always tilted at 23.5 degrees. The two Tropics (Cancer and Capricorn) are at Latitudes 23.5N and 23.5S; the two polar circles (Arctic and Antarctic) are at 66.5N and 66.5S (66.5 = 90 - 23.5).
With a calendar the numbers in the next table can be verified. The seasons referred to are Northern Hemisphere.
|Spring||92 days 19 hours|
|Summer||93 days 15 hours|
|Autumn||89 days 19 hours|
|Winter||89 days 01 hour|
As is well known, the Earth revolves around the Sun. Its path, however, is not a circle, but a curve called an ellipse. This means that the Earth's distance from the Sun varies throughout the year. Around 6 January, the Earth is at its closest to the Sun (147.0 million km) whereas around 5 July it is at its furthest (152.0 million km). The Sun appears slightly smaller in July so Total Eclipses of the Sun tend to be longest then.
Isaac Newton (he of the apple) showed that when a planet is closer to the Sun it travels faster. This is true for the Earth. This means that between October and April the Earth is moving faster around the Sun than between April and October. The reason for the speed changes (and I won't dwell on them here) is to do with the conservation of a property called angular momentum.
The 3% difference in our distance from the Sun causes us to receive 7% more energy during January than during July. You would expect that the seasons in the Southern Hemisphere to be more extreme than those in the North. But as it happens the Southern Hemisphere has more ocean than the Northern. This fact, and the presence of Antarctica, tend to cancel out the effect of the extra heat so that there is no real difference in the intensity of the seasons in the two hemispheres.
Just to baffle the reader completely, the point on the Earth's orbit where it is closest to the Sun is called Perihelion (more Greek). This point turns out not to be fixed in space. It moves forward around the Earth's orbit. This gives us a third type of year, the Anomalistic Year which is from Perihelion to Perihelion. Its length is 365 days 6 hours 13 minutes and 53 seconds, 4 minutes longer than the Siderial Year. In fact, the perihelion itself makes a complete circuit of the Earth's orbit in just under 3 million years! This motion has no terrestrial significance but is important to take it into account if you want to send a craft to Mars and return it safely.
The Solar Calendar was first used by the Sumerians. It is more difficult to measure because you cannot see the stars at the same time as the Sun in the sky. Observations have to be made indirectly at night. The stars visible at a certain hour each night vary throughout the year as the Sun and Earth change positions in space. However, the Solar Calendar is more accurate and less variable. The Greeks actually measured the length of both the Tropical and Sidereal Years and discovered the 20 minute difference between them. The main problem with the year is that it is not made up of an exact number of days.
Under Julius Caesar, the year was defined as 365.25 days. He divided the year into months (each with either 30 or 31 days alternating regularly) and invented the concept of a Leap Year. Each year had 365 days except Leap Years which had 366. This was the Julian Calendar.
Under the Romans the beginning of the year was moved from the Vernal Equinox to the beginning of January. The original 5th month (a 31 day month) was renamed July after Julius Caesar. Later, Augustus renamed the 6th month after himself (August) and increased its number of days to 31 to match July, leaving poor February with 28. It is Augustus we must blame for the awful rhyme we have to learn so that we can remember how many days each month has.
Unfortunately, the Julian Calendar contained three days too many for every 400 years. Slowly, the seasons began to get out of step. Eventually, it was decided to change the calendar. Previously all years divisible by 4 had been Leap Years. Pope Gregory declared that all century years would only be Leap Years if they were divisible by 400. In the new system 1900 was not a Leap Year but 2000 was. This had the effect of dropping 3 days every 400 years. To bring the calendar back in line with the seasons, 11 days had to be dropped from one year. This caused riots as people thought that their lives were being shortened! The Catholic countries followed the Pope and made the change during the 17th century, but the others delayed. The USSR made the change in 1917 after the Revolution by which time the error was 15 days. This is why the 'October Revolution' (October in the old style calendar) used to be celebrated in early November (new style)! This new calendar is called the Gregorian Calendar and is the one in use now. It is accurate to 1 day in 3000 years.
Sorry, but even this statement is not 100% true.
The Western Calendar that is in general use is said to begin from the Birth Of Jesus. In the year that was then known as "1288 years after the founding of Rome", a monk called Dionysus Exeguus decided to 'calculate when Jesus was born'. He studied the Bible and calculated that Jesus had been born 535 years earlier. About 250 years later, a European monarch, Charlemagne, decided it was better to count the years from the birth of Christ rather the founding of a pagan city. Thus, the Roman Year 753 became 1 AD. AD comes from a Latin phrase, Anno Domini ("in the year of the Lord").
There was no Year Zero because Europeans did not know about it. The zero had been invented in India but had not yet reached Europe. The West was still writing in zero-less Roman numerals (I, L, X, C, D, M, etc). The Western Calendar begins with year 1. This was followed by year 2, 3, etc all the way up to 2000. The years could also be counted backwards. Because zero was unknown, the year previous to 1 AD was 1 BC ("before Christ").
The Western Calendar has no year zero. This means that the first century ran from year 1 to year 100 inclusive. The second century ran from 101 to 200. Similarly, the first millennium ran from 1 to 1000. The second millennium covered the years 1001 to 2000 inclusive.
That means that the "first decade" of the "third millenium" ended on 31 December 2010.
Just to complicate matters, later historians have found that, if Jesus did exist, he would have been born before the death of Herod The Great. We now know the latter died in 4 BC. This gives the ridiculous statement that Jesus may have been born around 4 BC!
The Western Calendar is the world standard, used for all business, scientific and political transactions. Even so, there are different ways of writing dates. In most of the world (Europe, Asia, Africa, Latin America), 11th September 2001 would be written 11/09/01 but in North America this date would be written 09/11/01. This is a major problem with international media like internet web sites or email where the vast majority of the dates are written Month-Day-Year while the majority of the world uses Day-Month-Year.
The Western Calendar is not the only system in use. There are other calendars (some Lunar, some Solar) each with different zeros, marking events important to the people using these calendars.
For example, Muslims use the flight of Mohammed from Mecca to Medina as the start of their Lunar Calendar. This happened during the Western 7th Century AD so the Muslim Year is 1441 (until early September 2020). Buddhists use the birth of the Buddha so the year in Buddhist countries like Thailand is 2542. The Jewish Year is 5780 (until September 2020). The Chinese Year is 4717 (until Fenruary 2020). The Japanese Year is 2672. The Ethiopian Year is 2012 (until September 2020), the Persian Year is 1398 (until 21 March 2020). Under the old Roman calendar, the year is 2772 (until 14 January 2020).
Most of these calendars begin their years at different times by our calendar and that some New Years have fixed dates while others have movable dates (like Easter). Some calendars begin their day at sunset. The Western Calendar Day begins at midnight.
The important thing to realise is that calendars are human creations based on astronomical observations for their form and historical or cultural landmarks for their start points. These numbers have no significance in the Universe as a whole. If Charlemagne had not become king we might still be using the Roman Calendar. If Muslim forces had conquered Europe then the Muslim calendar might now be the world standard.
Now, if we lived on Mars or Venus ...
© 1997, 2017 KryssTal
The Calendar FAQ
Interesting discussion of calendars including details of calculating Easter and details of Roman, Jewish, Islamic, Chinese, and Mayan calendars.
International Date Formats
Proposals to standardise date and time notation for international usage.
The Equation of Time explained with animations and diagrams. Download a program that gives sunrise and sunset times for the whole year from any location.