Does The Equinox Have Equal Lengths of Day and Night?

After a long and dark Pacific Northwest winter, even the biggest storm and snow fans look forward to the longer days of spring. And perhaps no day is anticipated more than the spring equinox, which is, astronomically speaking, the first day of spring.

Yet, one of the biggest misconceptions in modern civilization has to be the idea that an equinox has equal lengths of day and night. It’s easy to see why – after all, “equinox” is Latin for “equal night,” and there’s no reason to doubt that the Romans – the ones who brought us concrete, gladiators, and tubas – would make such a simple miscalculation. But the name is indeed a misnomer. The day is always slightly longer than the night on an equinox, autumn or spring.

In this blog, I’ll give some “equinox 101” and introduce ya’ll to the equinox’s lesser-known cousin, the equilux. But in order to understand any of these things, we first have to understand the concept of the ecliptic.

What is the Ecliptic?

The ecliptic is the apparent path of the sun across the sky. The path is “apparent” because it is the Earth that is actually making this path (we just see it portayed on the sun because we are stationary observers on the Earth). The apparent path of the sun is generally not discernible to the everyday observer due to the Earth’s rotation and our resulting sunrise/sunset cycles. However, you can trace the ecliptic more easily at night by seeing which constellations are visible at different times of the year.

By definition, the plane of the ecliptic is the same as the Earth’s orbital plane around the sun. Additionally, all of the planets in the solar system lie very close to the ecliptic plane. Next time you are out stargazing and you see one of the planets, see if you can find the others in the ecliptic!

Now that we know that the Earth orbits the sun in the ecliptic plane, let’s properly define the equinox.

What is 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.

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.

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, as I stated earlier, the day is always longer than the night at an equinox. There are two reasons why.

1.) The Sun is a Disc

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

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.

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.

We just had our equilux yesterday (3/17) here in Portland, with our sunrise at 7:18 am and our sunset at 7:19 pm. With robins chirping, days longer than nights, and even some flooding from snowmelt on the Columbia River, I think it’s a pretty safe bet to say that spring has arrived in Portland. However, if you ask an astronomer, you’ll have to wait until 3:28 am, Monday, March 20th, when the celestial equator has intersected the ecliptic, for spring to truly have sprung.