Disclosure statement

Osnat Katz receives capital from the AHRC and the science Museum, and her collaborative doctoral programme is with the science Museum.

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The sun throughout the year. Shutterstock/Artreef

Earth is not like Venus. Instead, earth axis is knocked somewhere on its next – the technological name for this is “axial tilt”, and this tilt is responsible because that both the seasons and also the equinox. Together the earth spins top top its axis and also orbits the Sun, the intensity of the sunlight reaching various parts that the Earth’s surface ar changes. This is why we have seasons here on Earth.

You are watching: Day and night of equal length

Earth’s axial tilt also way that our planet’s equator is tilted loved one to the aircraft of that is orbit around the sun – what astronomers speak to the “plane of the ecliptic”. As soon as the center of the Sun’s disc perfectly crosses the equator, astronomers specify this together the equinox. This happens double a year, once in so late March and also once in so late September.

Not fairly equal

You might think the the lengths that the day and also night would be equal during the equinox. As it transforms out, this is only about true. The lengths the the day and night aren’t rather the same, and also there space two factors for this.


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earth tilt. Shutterstock/Dimitrios Karamitros

Firstly, the Sun has actually a dimension – that not simply a allude in the sky. This appears obvious, however it affects just how scientists measure sunrise and sunset. Technically, sunrise starts as soon as the top edge the the sunlight meets the east horizon, and also sunset ends as soon as the top edge the the sun sinks listed below the west horizon. Since the sunlight is no a point, and has upper and lower edges, this means that the equinox has a slightly much longer day than night.

Secondly, the Earth’s setting refracts (bends) sunlight. Once light overcome from one medium to another, its course changes. Sunshine travels through the vacuum of space, and also when it travels through Earth’s comparatively denser atmosphere, the bends.

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This bending method that we can see the top edge that the Sun number of minutes prior to it touches the eastern horizon, and also it also way that we deserve to see the top edge that the Sun number of minutes after the Sun has actually sunk under the western horizon. This add to even an ext time ~ above daylight throughout the equinox.

What’s more, the bending transforms depending top top the temperature and pressure that the atmosphere, for this reason the lengths that the night and also day top top the equinox are only ever approximately the exact same at any allude on Earth.

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There are days approximately the time of the equinox, where day and night room equal length. This are dubbed equilux, and when castle happens counts on latitude. In the UK in 2021, this occurred on 17 March.