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Equinox vs. equilux—when are day and night really equal?

Last Sept. 23, the world experienced its second equinox of the year. The first one happened on March 21. 

 “Equinox” comes from Latin words aequi, which means equal, and nox, which means night. Put together, they can be taken to mean “equal night.” Because of this, most people think that equinox is the time of the year when daytime and nighttime are exactly equal in length (12 hours each). However, this is not the case. The time of the year when daytime and nighttime are equal is a different day called the equilux.

There are two ways to view equinox. One is from outer space, and the other one is from our view in the ground. 

Viewed from space, equinox is the time of the year when the Earth’s axis is tilted perpendicular to the Sun. What this means is that during equinox, the axis of the Earth is neither tilted away from or towards the Sun.

This is due to the tilt of the Earth’s axis. Compared to its path around the Sun, the Earth’s axis tilts at approximately 23.5 degrees. 

This means that for half of the year, the north direction of the axis is tilted toward the Sun. During the remaining half, it is tilted away from the Sun. The two equinox days are the transition days between these two halves of the Earth’s path around the Sun. 

The tilt of the planet’s axis is the reason why the Earth experiences seasons. Different parts of the world get different amounts of sunlight during different times of the year.

Viewed from the ground, equinox is the time of the year when the Sun rises exactly east and sets exactly west. Given this, it is natural to expect that this is also the time of the year when day and night are exactly equal. However, there are several factors why daytime is actually longer than nighttime during equinox.

One reason why day is longer than night during equinox is the shape of the Sun. As seen in the sky, the Sun is not a dot or a point. Instead, it is a disk or a circle. This means that even if the center of the Sun spends 12 hours in the sky, the rest of the Sun is visible for more than that.

Imagine the Sun as a disk in the sky. If the center of that disk is just rising above the horizon, that means half of it is already above the horizon for some time. Meanwhile, if the center of that disk is setting below the horizon, the remaining half is still above the horizon. 

Another important factor is the presence of the Earth’s atmosphere. The gas that surrounds the Earth acts as a prism that bends light coming from outer space, similar to how water bends incoming rays of light. 

Because of this bending of sunlight, the Sun can appear in the sky even though it actually is below the horizon! This happens both before sunrise and after sunset. 

The Earth’s atmosphere also scatters the light coming from the Sun. This happens because tiny particles in the atmosphere are able to make sunlight bounce around. This is one of the reasons why there is twilight, which is that time of the night when the Sun has already set (or is yet to rise), but there is still some sunlight in the sky.

So, if equinox is not the time of the year when day and night are exactly equal, when are day and night equal then? In other words, when does equilux happen? 

The answer is a lot more complicated than for equinox. Equinox happens at the same time throughout the whole Earth. Meanwhile, equilux depends on location. 

When equilux happens also depends on a lot of other factors, including the presence of mountains which can make sunset come earlier and sunrise come later.

In the northern hemisphere, equilux occurs before the March equinox and after the September equinox. In Manila, equilux happened on March 12 and Oct. 2 of this year. Meanwhile in Cebu City, equilux happened on March 9 and Oct. 5. For comparison, Tokyo experienced equilux on March 17 and Sept. 27. In Sydney, which is in the southern hemisphere, experienced it on March 24 and Sept. 20.

Places close enough to the equator may not experience equilux at all. For example, the days in Singapore are always longer than the nights.

By the end of next week, most of the Philippines will have experienced equilux. (Davao City, for example, will have experienced it on Oct. 11.) From then on, the progressively shorter days will follow us all the way through the Christmas season many of us look forward to.

Topics: Sounds of Science , Equinox , equilux
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