Question

On which two days does the Sunrise exactly in the East?
A) April 21, July 23
B) March 21, September 23
C) March 23, September 21
D) June 21, December 22

Answer 1

Hint: Let us consider only one motion for the time being: The Earth's spin (or rotation) on its axis. The Sun, Moon, planets, and stars all rise in the east and move westward through the sky since the Earth rotates or spins toward the east.

Complete answer:
The rising and setting points of the sun vary significantly from day today. The Sun rises as far to the northeast as it ever does at the summer solstice, and sets as far to the northwest as it ever does. The Sun rises a little farther south every day after that. The Sun rises due east and sets due west on the autumn equinox.

Now let us look into the given options:
Option A) April 21, July 23: April comes after Vernal equinox and July comes after the summer solstice. Where sunrise is not exactly in the east in both cases, so this could not be correct. Hence, this option is incorrect.

Option B) March 21, September 23: The sun rises due east just two days a year, on the equinoxes, which occur near March 21 and September 22. It rises somewhere else every other day of the year. But where are they and why are they there? The tilt of the Earth as it revolves around the sun holds the answers.

Only when the circular course of our turn on Earth's surface breaks into two equal portions, half in the light and half in the dark, does the Sunrise and set exactly due east and west. The alignment occurs only at the spring and fall equinoxes because our planet's rotation axis tilts by 23.5° with respect to its orbital plane. However, there are two days in the year that the Sun rises exactly in the East, namely March 21 and September 23. Hence, this option is correct.

Option C) March 23, September 21: The sun has returned to the equator and is heading south about September 21. This is the only other day of the year where the sunrise and sunset are opposite each other. The Autumnal Equinox is what it's called. So, the sun does not rise exactly in the east on 23 March, 21 September. Hence, this option is incorrect.

Option D) June 21, December 22: From the summer solstice (June 21), the Sun moves southward, and from the winter solstice (December 22), the Sun moves northward. So, it is in the southeastern part during the summer solstice and in the northeastern part during the winter solstice. That is, the Sun rises 30 degrees north of due east and sets 30 degrees north of due west at the summer solstice. It rises at 30 degrees south of due east and sets at 30 degrees south of due west on the winter solstice. Hence, this option is incorrect.

Note: An equinox occurs when the plane of the Earth's equator intersects the geometric centre of the Sun's disc. This happens twice a year, about the 20th of March and the 23rd of September. In other words, it is the time that the apparent Sun's core is directly above the equator.