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Distance travelled ~ 429'657'600 km: day 167Ìý

In this blog post we are going to show you how you can calculate the exact tilt of Planet Earth by using your shadow. You can only do this at a certain time of the year, the Summer Solstice, and that moment is fast approaching. Ìý

This year Summer solstice is on 21 June. This is the longest day of the year and also for us here in the northern hemisphere, the time when the Sun is highest in the sky. Astronomers regard it as the start of summer for the northern hemisphere winter for the southern.

So what's the Summer Solstice got to do with measuring the tilt of the Earth, I hear you ask. Well, the orientation of the Earth to the Sun is defined by angles, and on June 21 the physics align so that you can use the position of the Sun in the sky at 1pm BST to accurately measure the tilt of the Earth. It still works around 2 days before and 2 days after so you have a few days to try this out.

Here's a tip - measure from the balls of your feet to the top of your shadow and remember be careful and don't look directly into the Sun.

The first important angle is Solar Zenith. This is the angle between the Sun and straight up and it's marked in green on the figure below. We can find this because the smaller this angle is - or the higher in the sky the Sun is - then the smaller your shadow. To work this out we use . If you know about tan, then your height is the adjacent side and your shadow is the opposite side of a square-angled triangle.

Figure 1.

Next we need latitude. This is essentially the angle between the equator and your position on the Earth. You can see from the figure above that if you take the green angles away from the pink angle then you get the yellow angle. This angle between the equator and the place on Earth where the Sun is straight above you is called the "solar declination". But on the solstice it's exactly the same as the tilt of the Earth - convenient.

How to find your latitude?
There are loads of ways of getting your lattitude -a gps device maybe on your phone or satnav, or endless websites can provide you with this. Some are as simple as putting in your postcode.

Most importantly the 23 Degrees team would love to hear how you get on. Send us your calculations, photos or videos of you doing the challenge on the Solstice, or maybe a photo of your shadow. Shadows have long been a key indicator to our Earth's position in it's orbit around the Sun.

Continue the conversation and put #solstice in your tweets!

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