Astronomy

Date of solstices pre-0 AD

Date of solstices pre-0 AD


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I've written a script based on Meeus's Astronomical Algorithms to calculate the position of the sun, moon etc. up to about 4000 BC. Everything looks good, except when I go to 4000 BC the solstice are about a month after expected dates (they fall mid-July and mid-January. I checked through this forum and came across this: /a/13009 . I looked at the results and saw that we have very similar dates.

Does the precession mean the solstices and equinoxes can shift to this extent? Or have I forgotten a step?

Many thanks.


Sounds like Julian Calendar slippage.

The Gregorian Calendar, the one we use now, was created to fix a problem with the Julian Calendar: The fact that the Solar Year wasn't exactly 365.25 days. As a result, compared to the calendar year, the date of the Vernal Equinox (and more importantly at the time, Easter) was slipping forward.

To remedy this, ten calendar days in 1582 (or up to thirteen calendar days in 1918, depending on how long a particular country waited to adopt the Gregorian Calendar) were omitted, and the Gregorian Calendar added a modification to the Julian calendar's rule of One leap year every 4 years: No leap year on century-years that aren't divisible by 400.

If your code automatically switches from Gregorian to Julian Calendar at the appropriate year, the adoption of the Julian Calendar probably winds up at about the expected days when the Julian calendar was adopted mid-first-century BCE.

But every 400 years further back, your calendar is slipping about three extra days.

And at 4000 BCE, that's thirty days.


The December Solstice and a Possible Christmas Star?

The December solstice this year is at 5:02 a.m. EST tomorrow, Monday, December 21. This will be the official start of winter in the Northern Hemisphere and the beginning of summer in the Southern Hemisphere. That day also will be the shortest day of the year in the Northern Hemisphere and the longest day in the Southern Hemisphere. For locations in the Northern Hemisphere, the noonday sun will appear its absolute lowest in the sky on the December solstice. Conversely, in the Southern Hemisphere the noonday sun will be at its absolute highest in the sky on the December solstice (except in the southern tropics).

The reverse is true on the June solstice (six months from now). On the June solstice, the sun will be at its northernmost point, with the noonday sun highest for Northern Hemisphere locations not in the tropics. In the Southern Hemisphere, the noonday sun will be lowest in the sky. The June solstice marks the official start of summer in the Northern Hemisphere and the beginning of winter in the Southern Hemisphere.

Halfway between the solstices are the March and September equinoxes. On the equinoxes, all regions of the earth except the poles experience twelve hours of the sun being above the horizon and twelve hours of the sun being below the horizon (the word equinox comes from the Latin for “equal night”). The equinoxes are the traditional beginnings of spring and autumn in most cultures.

Many people are surprised to learn that the sun doesn’t always rise due east and set due west. The sun rises due east and sets due west only on the equinoxes. Between the March and September equinoxes, the sun rises north of east and sets north of west. This is true worldwide, except near the poles. Conversely, between the September and March equinoxes, the sun rises south of east and south of west, also worldwide. On the December solstice, the sun rises as far south of east as possible and sets as far south of west as possible.


BIBLIOGRAPHY

E.M. Bruins and M. Rutten, Textes mathématiques de Suse, in the series Mémoires de la Mission Archéologique en Iran (Paris. 1961).

O. Neugebauer, Mathematische Keilschrifttexte, 3 vols., which is Quellen und Studien zur Geschichte der Mathematik, Astronomie und Physik, Abt. A, 3 (Berlin. 1935–1937). Cited as MKT.

O. Neugebauer and A. Sachs, Mathematical Cuneiform Texts, which is American Oriental Series. 29 (New Haven, 1945). Cited as MCT.

F. Thureau-Dangin. Textes mathématiques babyloniens (Leiden, 1938).

The Sexagesimal System , Before 2000 B.C. the Sumerians. the inventors of cuneiform script, who lived in southern Mesopotamia, used a positional system with base 60 for writing integers and fractions. “Positional” means that the value of a numeral depends on where it stands. The symbol for I, a vertical wedge, can denote any power of 60 or any power of 1/60. Integers up to 59 were written in the decimal system. Thus, 21 would be written as , the symbols for 10 and 1 being repeated as often as necessary. Beyond 59, integers were written as sums of multiples of powers of 60. Thus, 80 = 60 + 20 would be written as .

The normal way of writing fractions was to express them as multiples of 1/60, or of 1/60 2 , and so on. The denominator 60 n was not written: it had to be inferred from the context. Thus, 1 1/2 would be written as 1,30 and the reader would have to determine from the context whether I 1/2 or 90 was meant, or even 90 X 60 n or 1 1/2 X 60 -n .

After 2000 B.C. the Babylonian and Assyrian scribes inherited this system from the Sumerians. together with cuneiform script, which they adapted to their Semitic languages.

In transcribing Sumerian or Babylonian numerals, I shall follow the method introduced by O. Neugebauer, separating successive sexagesimal places by a comma:

If it is known that the first unit has the value 1, integers and fractions will he separated by a semicolon:

On the other hand, if it is known that the first unit has the value 60 2 . I shall write


Is the summer solstice on the same day every year?

No, although it is narrowed to just three days.

The summer solstice can occur between June 20 and June 22 each year in the northern hemisphere depending on when the Sun is directly over the Tropic of Cancer at midday.

This year's summer solstice began on Monday, June 21, 2021, at 4.31am BST.

In the southern hemisphere the date in June marks the winter solstice, or shortest day, when they have the least amount of daylight.


The Solstice is an astronomical event that happens twice, once in summer and once in winter, each year when the Sun reaches its highest position in the sky as seen from the North or South Pole. During Solstices the tilt of the axis of the Earth (with respect to the Sun) is the maximum at 23° 26'.

Solstices occur on 20 th or 21 st June and 21 st or 22 nd December each year. During summer the day of the solstice is the longest day of the year and during winter the day of the solstice is the shortest day of the year.

During June it is Summer Solstice in the Northern hemisphere and Winter Solstice in the Southern hemisphere. In other words, on June Solstice it is summer time in the UK, the USA, Canada, Russia, India, and China and it is the longest day of the year while it is winter time in Australia, Argentina, Chile, New Zealand and South Africa and it is the shortest day of the year.

Similarly, during December it is Winter Solstice in the Northern hemisphere and Summer Solstice in the Southern hemisphere. In other words, during December Solstice it is winter time in the UK, the USA, Canada, Russia, India and China and it is the shortest day of the year while it is summer time in Australia, Argentina, Chile, New Zealand and South Africa and it is the longest day of the year.

To avoid any confusion Solstices are preferably referred to as June Solstice (Northern Solstice) and December Solstice (Southern Solstice). Summer Solstice is also known as Aestival Solstice.

In Hindu astrology, Summer Solstice is known as Tropical Dakshinayana. However, Sidereal Dakshinayana starts from Karka Sankranti - from this day onwards Asurakal starts which is not good to start auspicious work.


Tilting planet

But what causes the winter solstice? Our planet has an axial tilt (of 23.4°) with respect to its orbital plane around the sun, which results in the seasons. The winter and summer solstices, and the vernal and autumnal equinoxes, are the extreme points in each of these seasons (see image). In winter, the Earth’s tilt away from the sun causes sunlight to be spread out over a larger surface area than in summer. It also causes the sun to rise later and set earlier, giving us fewer hours of sunlight and colder temperatures.

Meniou/Wikipedia , CC BY-SA

As it happens, the direction of the Earth’s tilt changes over time. These variations have been known about since the time of the ancient Greeks. Hipparchus, one of the founders of modern astronomical techniques, wrote one of the first comprehensive star catalogues in 129 BC. After compiling his catalogue, he noticed that the position of the stars had changed from those in much earlier records, such as the Babylonian.

Interestingly, the stars appeared to have moved position by the same amount, and he realised that the location of north in the sky must have moved in the intervening centuries. Currently, our celestial north is marked by the position of the star Polaris. But this was not always the case.

The rotation of a spinning object, like the Earth, can be affected by external forces. Given that the Earth is already spinning, any force applied to it, such as gravity from the moon or other bodies in the solar system, will modify this rotation (known as torque). The result on Earth is called the precession of the equinoxes – a phenomenon which affects our observations of the stars. A visible example of this on a smaller scale is shown several times during the film Inception, where the precession of a spinning top was used to determine whether the main character was in reality, or still dreaming.

For the Earth, this precession traces out a circle on the sky once every 26,000 years (see image below). In 3,000 BC, the celestial north was the star Alpha Draconis (Thuban), in the constellation Draco. Given that we can predict this motion, we know that 13,000 years from now our north star will be Vega, in the constellation Lyrae.

Author provided

This also affects the onset of the seasons over the length of a year as part of this 26,000 year cycle, and therefore has important implications for anyone attempting to attribute any cultural significance to a particular point in a given season. The time it takes for the Earth to orbit the sun is approximately 365.25 days, meaning we have an extra day every four years. By comparison, the precession of the equinoxes results in about 20 minutes of difference between the Earth’s orbital period when measured against the fixed background stars (a sidereal year), and the time it takes for the sun to appear to return to the same position in the sky each year (a solar year).

As a historical aside, it was the discrepancy between the length of the solar year and the length of a year as defined by the Julian calendar that prompted the conversion to the presently used Gregorian calendar. The precession of the equinoxes was known about and had caused a discrepancy of a few days which prompted the council of Nicaea to change our calendar system.

Under the Julian calendar, originally established by the Romans in 46 BC, New Year’s day in England used to be on March 25, and this was also used to define the start of the tax year. The adoption of the Gregorian calendar in 1752 shifted the date of the tax year forward by 11 days, but set New Year’s to January 1. However, to avoid 11 days of lost tax revenue, the government of that time set our tax year to begin on April 6 where it remains to this day.

So, given that there are 1,440 minutes in a day, and a difference of 20 minutes between the sidereal and solar years, then over a period of 72 years the dates of the equinoxes (and the solstices) would shift backwards in the calendar by a full day, if they were not corrected for (which they are). That means a Roman using the winter solstice as a reference point for the timing of Christmas would have been celebrating Christmas near the end of our November. Even further back, the builders of Stonehenge would have experienced the winter solstice in our September.


Solstice

Solstice,. During the course of a year, the sun appears to move northward for about six months and southward for about six months. The times when it reaches its northernmost point (on or about June 21) and its southernmost point (on or about December 22) are called the solstices.

On the day of the June solstice, the Northern Hemisphere has its longest period of daylight. At the Tropic of Cancer (2327' north of the Equator) the sun appears overhead at noon, and within the Arctic Circle the sun remains above the horizon all day.

On the date of the December solstice, the Northern Hemisphere has its shortest period of daylight. Within the Arctic Circle the sun does not rise above the horizon. The midday sun appears directly overhead at the Tropic of Capricorn (2327' south of the Equator).

In the Northern Hemisphere the June solstice is called the summer solstice, and the solstice in December, the winter solstice. In the Southern Hemisphere the seasonal names given the solstices are reversed.


B. Astronomical Measurements Without Instruments

    Heliacal risings: Helios is the Greek word for the Sun. Stars are said to exhibit "heliacal risings" if they rise in the east just before the Sun. An illustration is shown here. This is a (rough) method of tracking the Sun's changing position with respect to the stars. Recall that the Sun moves about 1 degree east every day against the stars. Hence this is a date-keeping method. For example, in ancient Egypt a heliacal rising of the brightest star, Sirius, was used to forecast the Nile's annual flood. The method can only be used for stars bright enough to be visible in the twilight sky.


A modern example of a horizon intercept

    Note: accurate Earth-sky angular measurements of this kind require establishment of a reference direction. For instance, two fixed points yielding a well-defined fixed line toward the horizon is a reference against which to measure anglular positions of intercepts. The two points could both be natural (e.g. a nearby rock and a tree on the distant horizon) or they could both be artificial --- the foreground road in the Moon image above, for example. The most sophisticated of the artificial reference systems were actually embedded into ancient buildings.

Many ancient building alignments were intended to mark the rise or set (i.e. the horizon intercepts) of important astronomical objects. Some examples:

    The Sun at the equinoxes (east-west alignment). For example, most ancient Greek temples have their long axes aligned east-west, so that the rising or setting Sun illuminates the interiors. The bases of the Egyptian pyramids are aligned almost exactly east-west/north-south, in the case of the Great Pyramid of Khufu within 3 minutes of arc (1/20 of a degree).

    The "El Castillo" pyramid, built by the Maya at Chichen Itza ca 950 AD, is not closely aligned E-W/N-S, but it features raised staircases against which a rippled shadow is cast by the edges of the pyramid at sunset near the equinoxes. See the image at the right. As the sun sets, the shadow moves and is said to resemble the slithering of the feathered serpent deity Kukulkan, to whom the temple is dedicated.


Events were watching for starting in November

We were on the lookout for the announcement of two major missions to space. The private company Moon Express’s attempt to put a lander on the moon to claim the $20 million Google Lunar X prize. And SpaceX demonstrating its Falcon Heavy rocket, an important step toward the company’s vision of sending people to space.

James S. Wood/Arizona Daily Star, via Associated Press

Crowds gather at Stonehenge for Solstice despite advice

People inside the stone circle during Summer Solstice at Stonehenge, where some people jumped over the fence to enter the stone-circle to watch the sun rise at dawn of the longest day of the year in the UK, in Amesbury, England, Monday June 21, 2021. The prehistoric monument of ancient stones have been officially closed for the celebrations due to the coronavirus lockdown, but groups of people ignored the lockdown to mark the Solstice, watched by low key security. Credit: Ben Birchall/PA via AP

Dozens of people have ignored advice not to travel to Stonehenge for the annual summer solstice celebrations, which were cancelled Monday due to coronavirus concerns.

English Heritage, which looks after the Neolithic monument, had planned a live feed of the sunrise at Stonehenge for the second year in a row. But the organization said that program had to be interrupted because of safety concerns after "a number of people have chosen to disregard our request to not travel to the stones this morning."

Thousands of people who tuned in to watch the sunrise at the stones online ended up watching pre-recorded footage before the live feed returned around 5 a.m.

Video from Britain's PA news agency and elsewhere showed dozens of people gathering inside the stone circle, with some scaling a low fence to climb inside the restricted area to reach the stones. Some were seen dancing and others held a banner that read "Standing for Stonehenge."

English Heritage said it was "disappointing" to see people "act in a way that put themselves, our staff and the police at risk" during a pandemic.

The summer solstice typically draws tens of thousands of people to the stone circle in southern England to celebrate the longest day of the year.

Wiltshire Police said despite a "minor incursion into the stone circle" early Monday, the solstice weekend was peaceful. One woman in her 50s was arrested in the area on suspicion of drunk and disorderly behavior, the force said.

  • A woman kisses one of the standing stones during Summer Solstice at Stonehenge, where some people jumped over the fence to enter the stone-circle to watch the sun rise at dawn of the longest day of the year in the UK, in Amesbury, England, Monday June 21, 2021. The prehistoric monument of ancient stones have been officially closed for the celebrations due to the coronavirus lockdown, but groups of people ignored the lockdown to mark the Solstice, watched by low key security. Credit: Ben Birchall/PA via AP
  • People inside the stone circle during Summer Solstice at Stonehenge, where some people jumped over the fence to enter the stone-circle to watch the sun rise at dawn of the longest day of the year in the UK, in Amesbury, England, Monday June 21, 2021. The prehistoric monument of ancient stones have been officially closed for the celebrations due to the coronavirus lockdown, but groups of people ignored the lockdown to mark the Solstice, watched by low key security. Credit: Ben Birchall/PA via AP
  • Police keep watch as people stand inside the stone circle during Summer Solstice at Stonehenge, where some people jumped over the fence to enter the stone-circle to watch the sun rise at dawn of the longest day of the year in the UK, in Amesbury, England, Monday June 21, 2021. The prehistoric monument of ancient stones have been officially closed for the celebrations due to the coronavirus lockdown, but groups of people ignored the lockdown to mark the Solstice, watched by low key security. Credit: Ben Birchall/PA via AP
  • People inside the stone circle during Summer Solstice at Stonehenge, where some people jumped over the fence to enter the stone-circle to watch the sun rise at dawn of the longest day of the year in the UK, in Amesbury, England, Monday June 21, 2021. The prehistoric monument of ancient stones have been officially closed for the celebrations due to the coronavirus lockdown, but groups of people ignored the lockdown to mark the Solstice, watched by low key security. Credit: Ben Birchall/PA via AP
  • A man touches one of the standing stones during Summer Solstice at Stonehenge, where some people jumped over the fence to enter the stone-circle to watch the sun rise at dawn of the longest day of the year in the UK, in Amesbury, England, Monday June 21, 2021. The prehistoric monument of ancient stones have been officially closed for the celebrations due to the coronavirus lockdown, but groups of people ignored the lockdown to mark the Solstice, watched by low key security. Credit: Ben Birchall/PA via AP
  • Security keep watch as people stand inside the stone circle during Summer Solstice at Stonehenge, where some people jumped over the fence to enter the stone-circle to watch the sun rise at dawn of the longest day of the year in the UK, in Amesbury, England, Monday June 21, 2021. The prehistoric monument of ancient stones have been officially closed for the celebrations due to the coronavirus lockdown, but groups of people ignored the lockdown to mark the Solstice, watched by low key security. Credit: Ben Birchall/PA via AP
  • People view the stones during Summer Solstice at Stonehenge, where some people jumped over the fence to enter the stone-circle to watch the sun rise at dawn of the longest day of the year in the UK, in Amesbury, England, Monday June 21, 2021. The prehistoric monument of ancient stones have been officially closed for the celebrations due to the coronavirus lockdown, but groups of people ignored the lockdown to mark the Solstice, watched by low key security. Credit: Ben Birchall/PA via AP

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