"Day" and "Night" are not of equal duration on Equinoxes!
Arvind Paranjpye, 23 Sep 2006
Contrary to
what is generally believed, days and nights are not of equal duration on
the equinoxes though the EQUINOX itself means the epoch on which and day
are of equal duration everywhere on the Earth.
The equinox
that occurs close to September 23rd is called autumnal equinox and the one
that occurs close to March 20th is called vernal equinox.
One can
confirm this easily by checking the time of sunset and sunrise published
in the almanacs or newspapers. Autumnal equinox for year 2001 takes place
on September 23, 2001 at 4: 34 IST. However, on 23rd in Delhi, the Sun
would rise at 6:10 a.m. and set at 6: 17 p.m. The day will be about 7
minutes longer than 12 hours. On the same day in Chennai, the sun would
rise at 5:58 a.m. and would set at 6: 04 p.m. making 12 hours and 6
minutes of duration in the day and 11 hours and 54 minutes of duration in
the night. As one continues to look at the these times on successive days,
one finds that on Sept 27 in Delhi, the day and night will be of 12 hours
each. In Chennai this will happen on Oct 2nd, which incidentally will be a
full moon night.
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Why day and
night are not of equal duration on the day of equinoxes is because the way
"day" hours are calculated. The Sun has an appreciably large size; it is
not a point object in the sky. The time of sunrise is calculated when its
upper edge (or limb, as we would say in astronomy) is seen above the
horizon and the end of sunset is defined when the last rays of the Sun
disappear below the horizon.
There is a further complication. Because of the bending, due to phenomenon
called refraction, close to the horizon sunrays are seen a few minutes
ahead or later of the "true" sunrise or sunset respectively. Thus, even
if the sun would have been a point object in the sky, we would not get
equal duration of day and night everywhere on the earth on a fixed date.
Interestingly, on the equator of the earth, the days are always longer
than the nights, all through the year by about 14 minutes.
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The material here can be used freely. It is however expected that the source will be acknowledged.
Credit : Inter-University Centre for Astronomy and Astrophysics, Pune.
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