果冻影院

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鈥檚 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鈥檚 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 鈥渆quinox鈥� comes from the Latin words听aequus听meaning equal and听nox听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鈥檚 tilt

The reason we on Earth have equinoxes at all is because the Earth鈥檚 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鈥檚 orbit around the sun, the intensity of the light shining on Earth鈥檚 hemispheres would be the same all year round, and we wouldn鈥檛 have seasons.

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

Earth is not like Venus. Instead, Earth鈥檚 axis is knocked somewhere on its side 鈥� the technical name for this is 鈥渁xial tilt鈥�, 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鈥檚 surface changes. This is why we have seasons here on Earth.

Earth鈥檚 axial tilt also means that our planet鈥檚 equator is tilted relative to the plane of its orbit around the Sun 鈥� what astronomers call the 鈥減lane of the ecliptic鈥�. When the centre of the Sun鈥檚 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鈥檛 quite the same, and there are two reasons for this.

Firstly, the Sun has a size 鈥� it鈥檚 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鈥檚 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鈥檚 comparatively denser atmosphere, it bends.

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鈥檚 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.

This piece was first published in on 18 March 2021.

Links

• Original article in听
• 果冻影院 Science & Technology Studies
• 果冻影院 Faculty of Mathematical & Physical Sciences

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