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[sci.astro] Time (Astronomy Frequently Asked Questions) (3/9)
Section - C.05 Was 2000 a leap year?

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Top Document: [sci.astro] Time (Astronomy Frequently Asked Questions) (3/9)
Previous Document: C.04 What's a Julian date? modified Julian date?
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Yes.

Oh, you wanted to know more?

The reason for leap days is that the year---the time it takes the
Earth to go round the Sun---is not an integral multiple of the
day---the time it takes the Earth to rotate once on its axis.  In this
case, the year of interest is the "tropical year," which controls the
seasons.  The tropical year is defined as the interval from one spring
equinox to the next: very close to 365.2422 days.

The Julian calendar, instituted by the Roman Emperor Julius Caesar
(who else? :), has a 365-day ordinary year with a 366-day leap year
every fourth year.  This gives a mean year length of 365.25 years, not
a very large error.  However, the error builds up, and by the
sixteenth century, reform was considered desirable.  A new calendar
was established in most Roman Catholic countries in 1582 under the
authority of Pope Gregory XIII; in that year, the date October 4 was
followed by October 15---a correction of 10 days.  Most non-Catholic
countries adopted this "Gregorian" calendar somewhat later (Great
Britain and the American colonies in 1752), and by then the difference
between Julian and Gregorian dates was even greater than 10 days.
(Russia didn't adopt the Gregorian calendar until after the "October
Revolution"---which took place in November under the new calendar!)
Many of the calendar changeovers elicited strong emotional reactions
from the populations involved; people objected to "losing ten (or
more) days of our lives."

The rule for leap years under the Gregorian calendar is that all years
divisible by four are leap years EXCEPT century years NOT divisible by
400.  Thus 1700, 1800, and 1900 were not leap years, while 2000 will be
one.  This rule gives 97 leap years in 400 years or a mean year length
of exactly 365.2425 days.

The error in the Gregorian calendar will build up to a full day in
roughly 3000 years, by which time another reform will be necessary.
Various schemes have been proposed, some taking account of the changing
lengths of the day and/or the tropical year, but none has been
internationally recognized.  Leaving a reform to our descendants seems
reasonable, since there is no obvious need to make a correction now.

User Contributions:

1
Keith Phemister
Sep 13, 2024 @ 11:23 pm
Copied from above: If the Universe were infinitely old, infinite in extent, and filled
with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP

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Top Document: [sci.astro] Time (Astronomy Frequently Asked Questions) (3/9)
Previous Document: C.04 What's a Julian date? modified Julian date?
Next Document: C.06 When will the new millennium start?

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