On which two dates would all locations on Earth have equal hours of day and night

Winter in the northern hemisphere and summer in the southern hemisphere are both coming to an end. That means the days and nights are becoming roughly equal in length, and the path the Sun traces across the sky is changing.

On a winter day, the Sun is low in the sky, whereas on a summer’s day the Sun lies considerably higher. But on a specific day in the spring or autumn, the Sun will be visible directly above the equator, somewhere in the middle of the two arcs traced by the Sun in the summer and winter.

This is what’s called the equinox, and there are two each year. Around 20 March we have the vernal equinox or March equinox, also known as the spring equinox in the northern hemisphere. Then around the 22 or 23 September is the autumnal or September equinox.

The word “equinox” comes from the Latin words aequus meaning equal and nox meaning night. But day and night are not exactly equal length on the equinox. To understand why, we need to know what causes the equinox in the first place.

Earth’s tilt

The reason we on Earth have equinoxes at all is because the Earth’s axis is tilted. The Earth spins around an imaginary line running through it called its axis. If the axis pointed straight from top to bottom, at a right angle to the direction of Earth’s orbit around the sun, the intensity of the light shining on Earth’s hemispheres would be the same all year round, and we wouldn’t have seasons.

Some planets in our solar system are like this – for example, Venus’ axis points nearly straight from top to bottom.

Earth is not like Venus. Instead, Earth’s axis is knocked somewhere on its side – the technical name for this is “axial tilt”, and this tilt is responsible for both the seasons and the equinox. As the Earth spins on its axis and orbits the Sun, the intensity of the sunlight reaching different parts of the Earth’s surface changes. This is why we have seasons here on Earth.

Earth’s axial tilt also means that our planet’s equator is tilted relative to the plane of its orbit around the Sun – what astronomers call the “plane of the ecliptic”. When the centre of the Sun’s disc perfectly crosses the equator, astronomers define this as the equinox. This happens twice a year, once in late March and once in late September.

Not quite equal

You might think that the lengths of the day and night would be equal during the equinox. As it turns out, this is only approximately true. The lengths of the day and night aren’t quite the same, and there are two reasons for this.

Firstly, the Sun has a size – it’s not just a point in the sky. This seems obvious, but it affects how scientists measure sunrise and sunset. Technically, sunrise starts when the upper edge of the Sun meets the eastern horizon, and sunset ends when the upper edge of the Sun sinks below the western horizon. Because the Sun is not a point, and has upper and lower edges, this means that the equinox has a slightly longer day than night.

Secondly, the Earth’s atmosphere refracts (bends) sunlight. When light passes from one medium to another, its path changes. Sunlight travels through the vacuum of space, and when it travels through Earth’s comparatively denser atmosphere, it bends.

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This bending means that we can see the upper edge of the Sun several minutes before it touches the eastern horizon, and it also means that we can see the upper edge of the Sun several minutes after the Sun has sunk under the western horizon. This adds even more time onto daylight during the equinox.

What’s more, the bending changes depending on the temperature and pressure of the atmosphere, so the lengths of the night and day on the equinox are only ever approximately the same at any point on Earth.

There are days around the time of the equinox, where day and night are equal length. These are called equilux, and when they happens depends on latitude. In the UK in 2021, this happened on 17 March.

There are only two times of the year when the Earth's axis is tilted neither toward nor away from the sun, resulting in a "nearly" equal amount of daylight and darkness at all latitudes. These events are referred to as Equinoxes. The word equinox is derived from two Latin words - aequus (equal) and nox (night). At the equator, the sun is directly overhead at noon on these two equinoxes.  The "nearly" equal hours of day and night are due to refraction of sunlight or a bending of the light's rays that causes the sun to appear above the horizon when the actual position of the sun is below the horizon.  Additionally, the days become a little longer at the higher latitudes (those at a distance from the equator) because it takes the sun longer to rise and set.  Therefore, on the equinox and for several days before and after the equinox, the length of day will range from about 12 hours and six and one-half minutes at the equator, to 12 hours and 8 minutes at 30 degrees latitude, to 12 hours and 16 minutes at 60 degrees latitude.

The Solstices (Summer & Winter)

The summer solstice occurs at the moment the earth's tilt toward/from the sun is at a maximum. Therefore, on the day of the summer solstice, the sun appears at its highest elevation with a noontime position that changes very little for several days before and after the summer solstice.  The summer solstice occurs when the sun is directly over the Tropic of Cancer, which is located at 23.5° latitude North and runs through Mexico, the Bahamas, Egypt, Saudi Arabia, India, and southern China.  For every place north of the Tropic of Cancer, the sun is at its highest point in the sky and this is the longest day of the year.

The winter solstice marks the shortest day and longest night of the year. In the Northern Hemisphere, it occurs when the sun is directly over the Tropic of Capricorn, which is located at 23.5° south of the equator and runs through Australia, Chile, southern Brazil, and northern South Africa.

For a complete listing of the dates of the winter and summer solstices and spring and fall equinoxes through 2025, check out this site from the U.S. Naval Observatory. An alternative text link is found here.

The Seasons

We all know that the Earth makes a complete revolution around the sun once every 365 days, following an orbit that is elliptical in shape.  This means that the distance between the Earth and Sun, which is 93 million miles on average, varies throughout the year.  During the first week in January, the Earth is about 1.6 million miles closer to the sun. This is referred to as the perihelion.  The aphelion, or the point at which the Earth is about 1.6 million miles farther away from the sun, occurs during the first week of July.  This fact may sound counter to what we know about seasons in the Northern Hemisphere, but actually, the difference is not significant in terms of climate and is NOT the reason why we have seasons.  Seasons are caused by the fact that the Earth is tilted on its axis by 23.5°.  The tilt's orientation with respect to space does not change during the year; thus, the Northern Hemisphere is tilted toward the sun in June and away from the sun in December, as illustrated in the graphic below.

The Relationship Between Length of Day and Temperature

There is a lag between the longest day of the year and the warmest average temperatures for most mid and high-latitude locations.

In northern OH/Northwest PA, the maximum daily temperature occurs nearly 3 weeks later in mid-July. Just as the warmest part of the day usually occurs several hours following noon, when the sun is highest in the sky, so too does the warmest part of the summer lags the summer solstice. This lag is due to the time required for ground and water to heat up. Average temperatures continue to climb until the sun drops lower in the sky. While the effect is evident in a daily temperature plot, it is more readily apparent by looking at changes in the monthly average temperature. In Cleveland, July averages 3.3°F degrees higher than June, with August also warmer than June by 2.8°F, even though the length of days in August is considerably less than the length of days in June.