Equal Day and Night? It Happens in June!

What are solstices and equinoxes?

Guayaquil’s 12-Hour Solstice

Every year, as people in the Northern Hemisphere celebrate the summer solstice, marking the longest day of the year and the beginning of summer, a select few will experience a period of equilux, when day and night are equal.

One such place is the northern half of Ecuador’s most populous city, Guayaquil. Here, day and night reaches a near-perfect 12-hour equilibrium each day for four or five days over the June solstice. (The city center, a few kilometers farther south, has an equilux a few weeks before the June solstice, and again a few weeks after.)

June Solstice Daylengths

What may surprise you is that Guayaquil isn’t on the equator but about 240 km (150 mi) south of it.

For Ecuador’s capital, Quito, which straddles the equator, the day of the June solstice lasts 12 hours and 6 minutes. In fact, Quito never experiences an equilux: each day of the year is longer than 12 hours.

Why is that so? Let’s find out.

Space Geometry

It’s one of those bits of folk wisdom you learn in school that starts to fall apart once you look at the fine print: that day and night are equal at the equinoxes.

After all, the word equinox is Latin for ‘equal night.’

Of course, there’s a kernel of truth to it. In late March and late September, the Sun is right above the equator, and Earth’s axis stands at a right angle to the Sun’s rays. Based on this simple fact of space geometry, every location on the Earth’s surface should get 12-hour days, right?

Solstice and equinox times down to the minute

Refraction and Sun’s Size = Longer Days

Wrong. For one, the claim refers to the geometric center of the Sun’s disk. But in reality, it has an angular size of about 0.5 degrees in the sky. A day begins and ends when its upper edge crosses the horizon, so the total length of day includes the time it takes for the upper half of the Sun to appear at sunrise and disappear at sunset.

Secondly, Earth's atmosphere causes refraction, making the Sun appear slightly higher in the sky, further increasing daylength. (The extent of this distortion depends on air temperature and pressure.)

Both phenomena result in slightly longer days, and they are the reason why

• equinox days are slightly longer than 12 hours all around the globe, and
• days close to the equator—including in Quito—are slightly longer than 12 hours all year round.

2.1 Degrees South: The Equilux Sweet Spot

For the timeanddate.com Astronomy Team, this inspired the question: How far from the equator do equiluxes start to occur, and when do they happen?

Delving into our Sun and daylength numbers for June, we found that sweet spot in Guayaquil, more precisely, at 2.1 degrees of southern latitude. (The longitude is irrelevant here.)

All locations closer to the equator experience no equiluxes; locations farther south have two equiluxes per year—and they happen on very different dates, as shown in graphic 1.

In the temperate zones, the equilux happens around, but never on the date of the equinox. For example, the equiluxes in New York occur on March 16 or 17 and September 25 or 26; in that time zone, the equinoxes fall on March 19 or 20 and September 22 or 23 each year.

A Perfect Balance

So, how come the zone experiencing equiluxes in the Southern Hemisphere begins at precisely 2.1 degrees of southern latitude?

Simply put, it’s the latitude where the day-shortening effects of Earth’s tilt in June (southern winter) and the cumulative day-lengthening effects of the Sun’s angular size and atmospheric refraction perfectly cancel each other out to produce 12-hour days at this time of the year.

Have a look at graphic 2 to see these effects in action from a top-down perspective:

• The Sun’s angular size and refraction cause the Sun’s light to reach slightly more than half the globe. In the illustration, this is represented by the slightly blurred transition between night and day.
• In June, the Southern Hemisphere is tilted away from the Sun. At 2.1 degrees south, this effect shortens the day just as much as the above-mentioned factors lengthen it: As the graph shows, the line marking the “real” transition between day and night—the left edge of the blurred zone—crosses that latitude at exactly its halfway point.

Northern Equilux Farther from the Equator

One of the fun facts that emerged during our deep dive into our Sun data—which took some additional detective work to figure out—is that the equilux sweet spot north of the equator is at a slightly higher latitude than in the Southern Hemisphere.

Here, the equilux zone starts closer to 2.2 degrees north (or, to be more precise, 2.17 degrees north). Locations on that latitude, such as Malacca in Malaysia, experience equal day and night on or around the December solstice, which marks the shortest day of the year and the beginning of winter in the Northern Hemisphere.

This very slight latitudinal difference, which only amounts to just over 7 kilometers (4 miles), is due to a very small change in the Sun’s angular size during the course of the year. “In December, Earth is a bit closer to the Sun, so the Sun is slightly larger in the sky,” explains Graham Jones, an astrophysicist at timeanddate.com.

“The effect of this tiny increase in the Sun’s angular size is that—all over the world—sunrise and sunset last a tiny bit longer. For the Northern Hemisphere, this means the minimum latitude at which an equilux can occur is a tiny bit further away from the equator.”