THIS Saturday (20 March) marks the vernal – or spring – equinox, when the sun crosses the celestial equator, sitting exactly between the hemispheres. It also marks the start of spring in the northern hemisphere and the beginning of autumn in the southern hemisphere.

You might have heard that day and night are of equal length around the world at the equinox (hence the name), but this isn’t strictly true. In an equinox, there are around 12 hours of daylight and 12 hours of night, but most places on Earth will receive slightly more daylight (see table).

Understanding why means thinking about sunrise and sunset. Sunrise occurs when the upper edge of the sun climbs above the eastern horizon, and sunset when its upper edge dips below the western horizon. The sun has technically risen even when only some of it is visible, and it hasn’t set until all of it has disappeared.

If we defined sunrise and sunset as the time when the centre of the sun rose above the horizon and then dipped away, we would have 12 hours of daylight at the equinox. But we don’t.

There are two reasons for this.First, the sun is a disc, not a point of light. This means that the middle of the sun can be below the horizon, but we still receive sunlight. On the equinox, the centre of the sun is visible for exactly 12 hours in a day. But there are a few minutes after sunset in which the rest of the sun is visible, giving us a slightly longer day.

The other reason concerns the refraction of light by Earth’s atmosphere. At sunset, for instance, the top of the sun stays visible for a few minutes after the sun has set, because the light is bent as it travels through the atmosphere. This effect adds 6 minutes to the length of the day.

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Wherever you live, the vernal equinox isn’t a day but a time. This year, it is at 9.37 am GMT on 20 March. If you want to celebrate it, take a look at a sunset this week and think about seeing the sun even when it isn’t actually there.

Although day and night aren’t the same length on an equinox, there are days around an equinox when day and night are very close to 12 hours each. These days are called the equilux, but when they happen depends on your latitude.

Just 5 degrees north of the equator, equilux happens on 24 February, compared with 18 March at 60 degrees north. Likewise, day and night are of almost equal length on 14 April just 5 degrees south of the equator, compared with 60 degrees south, where it happens on 22 March.