Earth May Have Been Shaped Like a Donut

Earth May Have Been Shaped Like A Donut At One Point In Time[1]

The new synestia theory proposes a new type of planetary object and another theory for how the Moon formed.

A to-scale illustration of the synestia process is shown above.

Astronomers have proposed a new type of planetary object they are calling synestia, where a celestial body violently collides with another body, resulting in a donut-shaped disk of vaporized rock. After some time, the body will cool down and turn into the solid, round planets we currently know.

Sarah Stewart, a planetary scientist at the University of California Davis, and Simon Lock, a graduate student at Harvard University in Cambridge, co-authored the study that was published in the Journal of Geophysical Research: Planets.

The name synestia combines the prefix “syn-” meaning “together” and Estia, the Greek goddess of architecture and structures.

The idea was modeled after ice skaters doing a spin—when they put their arms out to the side they slow down but when they tuck their arms in, their momentum stays constant, but their angular velocity increases and they spin faster.

The team was interested in how that idea would apply to rapidly spinning planets colliding with each other and how it would impact the angular momentum.

The people doing the study believe the collision that formed Earth likely caused a synestia before it cooled back down and formed into the solid, round object it is today. The researchers said those days probably didn’t last more than 100 years, though.

On top of a theory for planets how planets have formed, the study also lends to the idea of the formation of the Moon since its composition is similar to that of Earth’s.

[1] See Nicole Kiefert, Astronomy.com, “Earth May Have Been Shaped Like A Donut At One Point In Time,” (published May 30, 2017), downloaded May 30, 2017

Tropical Year

Tropical Year[1]

Recently we presented an article on the six definitions of a year. A year is not necessarily a year. “Annual” means yearly, but what is a year? There are actually 6 different years: Anomalistic year, Draconitic year, Julian year (this one you are familiar with — 365.25 days), Luna year, Sidereal year, and Tropical year.

A tropical year is the time it takes for the Sun to travel 360° along the ecliptic, its apparent path through the sky. Since the Sun doesn’t actually go around Earth, this should be restated as the time it takes Earth to complete a full orbit around the Sun. It varies from year to year, but to 8 significant figures it is 365.24219 days. The exact length of a tropical year can vary by up to around half an hour. For instance, the tropical year 2032 will last longer than 365 days and 6 hours. 2027, however, will only last 365 days, 5 hours, and 39 minutes.

 

The chart below shows the lengths of each of the years in days. Note how much shorter than the Calendar Year is the Sidereal Year—just a

Draconitic 346.620075883
Lunar 354.370000000
Tropical 365.242189000
Calendar 365.250000000
Sidereal 365.256363004
Anomalistic 365.259636000

It surprises people that our common conception of how to measure time is not precisely fixed, and there is the question: “A year is a year is a year?”

[1] Timeanddate.com

Sidereal Year

Sidereal Year[1]

Recently we presented an article on the six definitions of a year. A year is not necessarily a year. “Annual” means yearly, but what is a year? There are actually 6 different years: Anomalistic year, Draconitic year, Julian year (this one you are familiar with — 365.25 days), Luna year, Sidereal year, and Tropical year.

A sidereal year is the time Earth takes to make one orbit relative to the stars.

An Earth day can be measured in different ways.

  1. Measure the time it takes for a complete rotation of Earth around its axis.
  2. Measure the time it takes for the Sun to appear in the same meridian in the sky. This interval is known as the solar day.
  3. Measure the time it takes for a distant star to appear in the same meridian in the sky. This interval is known as the sidereal day. The time interval mentioned in (1) is equal to the sidereal day. Why? Because we measure the angular rotation speed by using distant stars as references. These reference points are very distant astronomical objects called quasars.

1 complete rotation of Earth around its axis takes 23 hours, 56 minutes and 4.1 seconds

Sidereal day is also 23 hours, 56 minutes and 4.1 seconds by definition, and is longer than the Solar Day.

Remember, Earth rotates around its axis eastward (counter-clockwise as seen from above the North Pole) and moves around the Sun at the same time. Earth moves around the Sun counter-clockwise as seen from above the Plane of the Ecliptic. The plane on which all planets of the Solar System revolve around the Sun (Pluto was an exception but it is no longer considered a planet) is known as the Plane of the Ecliptic.

Earth moves a little less than a degree around the Sun during the time it takes for 1 full axial rotation. So, for the Sun to appear on the same meridian in the sky again after 1 full axial rotation, the Earth has to rotate one extra degree to bring the Sun into the same apparent meridian in the sky. This is why the Solar Day is longer than the Sidereal Day by about 4 minutes.

The chart below shows the lengths of each of the years in days. Note how much shorter than the Calendar Year is the Sidereal Year—just a

Draconitic 346.620075883
Lunar 354.370000000
Tropical 365.242189000
Calendar 365.250000000
Sidereal 365.256363004
Anomalistic 365.259636000

It surprises people that our common conception of how to measure time is not precisely fixed, and there is the question: “A year is a year is a year?”

[1] Posted on April 26, 2014 by Suresh Emre

Lunar Year

Lunar Year[1]

Recently we presented an article on the six definitions of a year. A year is not necessarily a year. “Annual” means yearly, but what is a year? There are actually 6 different years: Anomalistic year, Draconitic year, Julian year (this one you are familiar with — 365.25 days), Luna year, Sidereal year, and Tropical year.

A lunar year or lunar calendar is one that is based on the cycles of the moon phases. The problem with a lunar calendar is that it drifts away from the seasons. Each year, the start and end dates of each month drift by 11 days. In order to stay correct, every lunar calendar has to deal with this drift away from the calendar year.

Lunar New Year, was called Chinese New Year, Tet (in Vietnam), and in other countries they would have another name for it. I will stick with Lunar New Year, so we could include all the countries who celebrate it. In China, Chinese New Year is also known as “Spring Festival”.

Let’s examine a year. A lunar month lasts 29.53 days. So after 12 lunar months, you’re about 354 days. This is short of the 365 days that it takes the Earth to orbit the Sun. This is a problem since after about 3 years, the lunar months are out of cycle with the solar year by about a month. And this problem would just continue.

To make the lunar calendar work in China, farmers would add in a leap month every 3 years. This would mostly get the lunar month to line up with the solar year, but they still drifted apart somewhat. For some calendars used for religious purposes, such as the Islamic Hirji calendar, they never bothered to sync up the calendars and let them drift. It takes 33 years for the cycle of lunar years to get back to the original position.

A lunar calendar was used in England up until Tudor times.

The chart below shows the lengths of each of the years in days. Note how much shorter than the Calendar Year is the Lunar year—12 days (and some minutes and seconds.)

Draconitic 346.620075883
Lunar 354.370000000
Tropical 365.242189000
Calendar 365.250000000
Sidereal 365.256363004
Anomalistic 365.259636000

It surprises people that our common conception of how to measure time is not precisely fixed, and there is the question: “A year is a year is a year?”

[1] Sun, Moon and Stars (February 26, 2013) and Astronomy (January 2016, p. 12)

Draconitic Year

Draconitic Year[1]

Recently we presented an article on the six definitions of a year. A year is not necessarily a year. “Annual” means yearly, but what is a year? There are actually 6 different years: Anomalistic year, Draconitic year, Julian year (this one you are familiar with — 365.25 days), Luna year, Sidereal year, and Tropical year.

The draconitic year, sometimes called the eclipse year, is the time it takes for the Sun to move from one lunar node (where the Moon’s path intersects the Sun’s) to the same lunar node from our perspective.

Because the paths of the Sun and Moon are at an angle to each other, the moon crosses the path of the Sun twice, each time the Moon goes once around Earth, once every month.

These points of crossing are called, lunar nodes, and they precess gradually westward, performing a complete circle in approximately 18.6 years: a draconitic or nodical period.

The Draconitic year is actually 346.620075883 days. (Yes, we can measure it that precisely, out to 9 places, or to 0.55629 seconds!)

The chart below shows the lengths of each of the years in days.

Draconitic 346.620075883
Lunar 354.370000000
Tropical 365.242189000
Calendar 365.250000000
Sidereal 365.256363004
Anomalistic 365.259636000

It surprises people that our common conception of how to measure time is not precisely fixed, and there is the question: “A year is a year is a year?”

[1] Sun, Moon and Stars (February 26, 2013) and Astronomy (January 2016, p. 12)

Anomalistic Year

Anomalistic Year[1]

A year is not necessarily a year. “Annual” means yearly, but what is a year? There are actually 6 different years: Anomalistic year, Draconitic year, Julian year (this one you are familiar with — 365.25 days), Luna year, Sidereal year, and Tropical year.

The Anomalistic year is the time Earth takes to travel from one perihelion (closest to the Sun) to the next perihelion. This is shown in the figure above. All the years might be the same if everything moved in gear-like fashion, and the Sun were fixed.

The Anomalistic year is actually 365.259636 days. Now that isn’t Earth shaking to be sure, but it is slightly longer than the Julian year we are used to, by 0.009636 days.

The chart below shows the lengths of each of the years in days.

Draconitic 346.620075883
Lunar 354.370000000
Tropical 365.242189000
Calendar 365.250000000
Sidereal 365.256363004
Anomalistic 365.259636000

It surprises people that our common conception of how to measure time is not precisely fixed, and there is the question: “A year is a year is a year?”

[1] Cosmos – The SAO Encyclopedia of Astronomy,