Does the Sun Rise in the East?

4 mins read

Since our childhood we are told that the sun rises the East and sets in the West. Such a trivial celestial phenomena this is that many students actually never question it.

Does it really rise in the East and set in the West (exactly)? If so, how does it happen on all days of the years or only on certain days? We will see this aspect and try and see some new insights.

Short Answer

The Sun rises due exactly east and sets due exactly west on only two days of every year.

Sunrise and sunset happens because Earth spins, counter-clockwise, if we look down from the North Pole. 

The earth spins on its own axis like a top, toward the direction we have named east, so from our perspective wherever we are on most of the earth’s surface (the poles are a little different in the details), the sun always comes into view on the eastern horizon as we spin in that direction, seen from our individual location. And as our part of the earth continues to spin toward the east, the sun appears to pass overhead and eventually out of sight past the western horizon.

The Sun rises and sets exactly due east and west only when the circular path of our turn on Earth’s surface splits into two equal parts, half in the light and half in the dark.

Our planet’s rotation axis is tilted by 23.5° with respect to its orbital plane, hence this alignment happens only at the Spring and Autumnal equinoxes.

During an equinox, the plane separating Earth’s day and night sides contains both the North and South Poles. On any days other than the equinoxes, this plane is askew, and our circular path of rotation passes unequally through Earth’s lit and dark sides. Therefore, the lengths of night and day vary, as does the position of the Sun’s rise and set on the horizon.

In all other days of the year it rises and sets with a component in North or South Axis (depending on the direction of earth axis relative to the normal of the ecliptic plane) – if you live exactly on north or south pole then it is a different case.

The total variation in degrees is 46.52 degrees between two extreme directions of the sun set or rise during the year. It does not depend on the latitude of the observer.

Experiment

Take a compass and measure the direction of sun set/ sun rise in the following dates: December 21, March 20, June 21, September 21.

What you will notice is that in March 20 and September 21 (equinoxes), the sun rises/sets aligned perfectly with east-west axis. But in December 21 and June 21 (solstices) the sun rises/sets with a very significant deviation from east/west (+ or – 23.6 degrees) – thats a big deviation from true East/West.

So, answering the question more directly, the rise and set phenomena occurs, due to earth’s rotation. Since it rotates from west to east (from a referential observer outside earth with the head aligned with north-south axis, pointing north, looking to earth), the sun appear for us as rotating upon the sky from east to west.

As we see it

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