The Equinox

The equinox is the  bi-annual occurrence where the sun shines directly over the Earth’s equator. More specifically, it is when the celestial equator intersects the ecliptic. The celestial equator is simply the “real” equator projected out to space. Earth’s equinoxes generally occur on March 20th and September 22 or 23, with year-to-year differences due to our discrepancies between a calendar year and a solar year.

Diagram of relationship between the equinox and the ecliptic/equatorial planes. The two equinoxes occur when the equator intersects the ecliptic.
Credit: Wikimedia user Tfr000

On other planets, equinoxes can appear quite dramatic. Saturn is a prime example – during an equinox, the sun is directly over the rings (because the rings are directly above the equator). As a result, light from the sun doesn’t illuminate the rings from above or below – it hits them edge-on and leaves them dark. The only illumination they get is from Saturn itself.

Saturn as viewed from the Cassini Spacecraft during August 2009, its last equinox. Because the rings are located over the equator, they are pointed nearly parallel to the Saturn’s orbital plane during an equinox, and therefore receive very little sunlight.
Credit: NASA

When the sun is directly above the equator like it is during an equinox, it spends the same amount of time above and below the horizon at every location on a planet. However, the day is always longer than the night at an equinox. There are two reasons why.

1.) The Sun is a Disc

Sunrise and sunset definitions

Sunrise and sunset definitions

Unlike every other star in the sky, the sun is close enough that it is visible to us as a disk of light instead of a single point. Sunrise and sunset occur when the top of the sun first rises or last sets above/below the horizon, even though the center of the disk is below the horizon in both these cases. This adds a couple extra minutes to our day.

2.) Refraction From Earth’s Atmosphere

Credit: RASC Calgary Centre
Author: Larry McNish

The Earth’s atmosphere refracts sunlight. This can make the sun appear above the horizon even after it has moved below it. Combining this with the fact that the sun is a disc, not a point, of light, lengthens the day by approximately 14 minutes near the equator. This number increases as you head towards the poles.

The Equilux:

The equilux is the day of the year with (approximately) equal day and night. Unlike the equinox, which is a fixed date for everybody, the date of the equilux is dependent upon the latitude of the observer. Between the poles and about 20 degrees latitude, it is generally a few days before the spring (or vernal) equinox or a few days after the autumn equinox.

However, once you get below 20 degrees latitude, the equilux much occurs earlier (later) than the spring (fall) equinox. Once you get within a couple degrees of the equator (more specifically, where the changes in the length of day throughout the year are less than 7 minutes), there is no equilux! This is because the day is ALWAYS longer than the night at these low latitudes.

Hours of daylight as a function of latitude and day of the year

Hours of daylight as a function of latitude and day of the year.
Credit: Wikimedia User Cmglee

The chart below shows the length of the day as a function of latitude and the day of the year and is based off the sunrise equation. The 12-hour contour marks the equilux.

Written by Charlie Phillips – Last updated 12/1/2017