Equinox day

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.

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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About skytonight

I am the present Director of Nehru Planetarium of Nehru Centre Mumbai, India I like to talk about astronomy and sky observations to general public.
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