Exploring the Mystery of Months & Days

[Janus]

We already covered that back in post #30. The Sidereal year is 365 days, 6 hr, 9 min, 9.7676 sec long. Thus in a non-leap year. the Earth will be 6 hr, 9 min, 9.7676 sec short of a complete orbit the next Feb 19. This works out to just about 1/4 of a degree or half the width of the Moon.

However, during a leap year, which is 366 days long, the year is longer than it takes for the Earth to complete an orbit by some 3/4 of a degree. So what you would get is the Earth falling behind by 1/4 of a degree for each of 3 years and then making that up in the fourth year. There will still be a minor drift caused by the difference between Tropical and sidereal year (again, as noted in post 30), but this would take years to notice.

 

[Integral]

We already covered that back in post #30. The Sidereal year is 365 days, 6 hr, 9 min, 9.7676 sec long. Thus in a non-leap year. the Earth will be 6 hr, 9 min, 9.7676 sec short of a complete orbit the next Feb 19. This works out to just about 1/4 of a degree or half the width of the Moon.

However, during a leap year, which is 366 days long, the year is longer than it takes for the Earth to complete an orbit by some 3/4 of a degree. So what you would get is the Earth falling behind by 1/4 of a degree for each of 3 years and then making that up in the fourth year. There will still be a minor drift caused by the difference between Tropical and sidereal year (again, as noted in post 30), but this would take years to notice.

This last bit of drift is compensated for with the century leap years, that is a century year (1800, 1900 etc. ) is NOT a leap year even though it is obvioulsy a multiple of 4, unless it is still divisible by 4 after dividing by 100. Thus the year 2000 was a leap year making it part of a 400 year correction cycle. I somehow feel cheated because the rare event that occurred in 2000 meant that we maintained the leap year cycle that we have all become familiar with. Since the current calendar system was established in the 1750's this is the first century which was a leap year.

 

[Janus]

This last bit of drift is compensated for with the century leap years, that is a century year (1800, 1900 etc. ) is NOT a leap year even though it is obvioulsy a multiple of 4, unless it is still divisible by 4 after dividing by 100. Thus the year 2000 was a leap year making it part of a 400 year correction cycle. I somehow feel cheated because the rare event that occurred in 2000 meant that we maintained the leap year cycle that we have all become familiar with. Since the current calendar system was established in the 1750's this is the first century which was a leap year.

Actually, I was referring to the difference between Tropical and Sidereal years. This drift is what is left over after all the calendar manipulations, which are designed to keep the calendar in step with the Tropical(seasonal) year. In other words, the drift due to the precession of the equinoxes. It works out to be about 1.4° per century.

 

[mreq]

So, Janus, if I want to calculate the sideral years from the point of year 0 how to do it ?
Thanks!

 

[Janus]

Our calendar has no year zero. But if you want to figure out how many sidereal years occur between the same date in different calendar years, you would:

Multiply the number of years between the dates by 365.
Add in the number of leap days that occurred during the period.
Divide this by 365.256363051

 

[mreq]

Janus please if you know another software, because Skyglobe it's not running on my pc.
Thanks!

 

[Chronos]

In ancient times, people relied on lunar cycles. 13 lunar cycles [28 days] equals 364 days. That was accurate enough unless your civilization persisted for centuries. After a few hundred or so years, you realized this clock was just a hair off [assuming you trusted your ancestors]. The Romans had this thing about the number 13, so they 'fixed' the calendar - probably just to annoy the encroaching barbarians.