Equinox days – equal or otherwise

It has been taught and generally believed that on the day of EQUINOX  daytime hours and nighttime hours are of equal duration. The EQUINOX itself means the epoch on which duration of day hours are of equal the duration to night hours, everywhere on the Earth.  The reason given is that on this day he sun-rays indeed fall perpendicular to the axis of the Earth.  Which, indeed, is quite true.

The one that occurs close to March 20th is called vernal equinox. And the other one that occurs close to September 23rd is called autumnal equinox (in the northern hemisphere of the Earth).

However, on days of equinoxes (occurring close to Mar 20 and Sep 22) nowhere on the Earth the duration of daytime and nighttime are equal.

Let us understand ‘why’ daytime hours and nighttime hours are not equal, everywhere, on the Earth on these days.

The reason why the duration of daytime hours and nighttime hours are not equal, on the same day everywhere on the Earth is for two reasons.  One is that the Earth is a spherical body. And, therefore, even though on the day of equinox the sun-rays fall perpendicular to the axis and therefore on the equator too, elsewhere else the rays fall at slanting angles.  And, therefore, the sunrise and sunset times would differ from one latitude zone to the other.

Secondly the Sun is not a point source. It has an appreciably large size of about half a degree. The time of sunrise is calculated when its upper edge (or limb, as one would say in astronomy) is seen above the horizon and the end of day when the last rays of the Sun disappears.

There is a further complication. Because of the bending of sunrays in atmosphere of the Earth, 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.

The cumulative effect of these factors is that on the equinox days the day hours are about 7 to 8 minutes longer than night hours. Near and at the poles this difference is much higher. At the pole the rays are almost parallel to the horizon. And, therefore, at both the poles the day would be 24 hours long.

Interestingly, on the equator of the Earth all through the year the day hours are always longer than the night hours by about 14 minutes. 

Having said this we would also like to tell you that, however, there are days when daytime hours equal nighttime hours indeed.  Such days are different for different latitudes.

For example people on latitudes of Delhi (~28 deg N)  and Chennai (~13 deg N)would have March equinox on 16th and 12th March and September equinox on 27th Sept and 2nd Oct. respectively.

One can confirm the fact easily by checking the time of sunset and sunrise published in the newspapers, almanacs or downloading almanac generating apps.



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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