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Astronomy at a distance – Sundials

making a sundial
The largest and most recognisable star in our sky is, of course, our own Sun. For obvious reasons, there is no point in searching for it at night and you should never look directly at the Sun, so if you want to find out more about it you need a tool that you can use during the day. Professor Tom Marsh from the Astronomy and Astrophysics Group explains how to make a sundial and what it can tell us about our Sun.

A sundial is a device for telling the time. They are easy and fun to make for yourself and you will be able to work out the North-South direction at your location as well. Be sure to try this only on a sunny day. Don’t look at the Sun because that can damage your eyes: you will only need to see where shadows cast by the Sun lie.

Equipment

You’ll need:

Some way to tell the time (e.g. clock, watch, phone)

Outside: A stick and some pebbles or any sort of marker that can be placed on the ground.

Inside: A piece of paper, two pens or pencils and some way to stand one of them upright.

Location: Anywhere that will get the Sun for as long as possible during the day. Somewhere away from the shadows of buildings or trees outside, or a sunny windowsill inside.

Method

Outside: place the stick vertically in the ground. It doesn’t need to be done precisely, but make sure it can’t move. Each hour, mark the end of the shadow cast by the stick with a pebble or other marker.

Inside: prop one of the pens or pencils vertically on a flat piece of white paper. The pen and paper must be fixed securely so that they won’t move. Make sure the shadow cast by the pen or pencil falls on the paper and that it will continue to do so for as long as possible as the Sun rises and sets (remember, from the UK, the Sun always moves towards the right during the day). Each hour mark the end of the shadow cast by the pen or pencil with a cross on the paper using the other pen or pencil.

Once you are done, then on any other day, you only need to look for where the shadow is compared to your markers to work out the time of day. Try this to see how accurate your sun clock is.

Finding North

The shadow of your stick or pen will be shortest at midday (close to 13.00 British Summer Time, BST, i.e. the time used after the clocks move forward on March 29). At this time the shadow points due North if your stick or pen is straight and vertical.

Taking things a step further

The exact time of midday varies depending on how far East or West you are in the country. Also it wanders relative to 13.00 BST because Earth’s orbit around the Sun is not exactly circular.

If you can keep your sundial set up for a long time over the spring and summer, you might see this through the time indicated by your sundial drifting compared to clocks and watches, and you should also notice that the length of the shadow cast by your stick, pen or pencil at midday will vary as the seasons progress, being shortest at the mid-summer solstice on June 20. If you can do this, try to keep records, e.g. of the time each day according to your clock or watch when your sundial indicates that it is midday.

Published:

12 June 2020

About:

The Astronomy and Astrophysics group at Warwick is interested in a huge range of scales across the Universe: planetary systems, how they form, live and die; stars, stellar binaries and and the exotic physical processes that they allow us to explore; as well as the transient events which mark the end of stellar lifetimes and the galaxies stars inhabit across the Universe. The group started in September 2003 and is both an observational and theoretical group. The group makes use of a wide range of ground-based telescopes, such as ESO's Very Large Telescope (VLT) in Chile and the Isaac Newton Group of telescopes (ING) in the Canary Islands, or the Atacama Large Millimetre Array (ALMA), as well as space telescopes such as NASA's Chandra and ESA's XMM-Newton X-ray observatories and the Hubble Space Telescope. The Warwick astro group partners in the four large spectroscopic surveys (DESI, SDSS-V, WEAVE, and 4MOST) that will start operations throughout 2020-2021.

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The text in this article is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).

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