Cool Sun Facts · Measure the Sun
Measure the Sun, with a stick and a clock
Four experiments that check this site's calculations against your own measurements, and a notebook that keeps them, with their uncertainties, in this browser.
Safety first
- Every experiment here uses a shadow, a patch of light, a skyline or a picture. None of them needs you to look at the Sun, and you should not.
- NASA: regular sunglasses, however dark, are not safe for looking at the Sun, and except during the brief total phase of a total eclipse it is not safe to look at it without special solar viewers.
- Binoculars, telescopes and camera lenses need a solar filter attached to the front; eclipse glasses must not be used with them.
Stick and shadow: how high is the Sun?
- Stand a straight stick upright on level ground. A weight on a string hung beside it shows true upright.
- Measure the stick's height above the ground and its shadow from the foot to the tip, in the same unit, and say how sure you are of each.
- Note the time to the minute, and say how sure you are of the clock.
Shadow series: find solar noon
Leave the stick where it is and measure its shadow every 15 or 20 minutes around midday. The shortest shadow comes when the Sun is highest, at solar noon, which is rarely 12:00 on the clock.
| Local time | Shadow length | ± (same unit) | Remove |
|---|
Horizon crossing: when does sunlight reach you?
Note the moment direct sunlight first reaches your spot over the hills or buildings, or the moment it leaves. Watch the light, not the Sun: your own shadow appearing on the ground, or a wall going from shade to sunlit. With a saved horizon profile for the place, the result is compared with its estimate as well as with standard sunrise or sunset.
Sunspot tracking: watch the Sun turn
On the Solar observatory, choose Visible light and find a sunspot in two images a day or more apart. For each, note the image's time (UTC) and the spot's position: across, from −1 at the left edge through 0 at the centre to 1 at the right edge, and up, from −1 at the bottom to 1 at the top.
Your notebook
Kept in this browser only, with your measurements exactly as you entered them. Recalculating changes the predictions, never the measurements.
Nothing saved yet. Work out an experiment above and press Save to notebook.
Without coordinates, an exported record keeps its place's name and time zone and every value worked out at the time, but cannot be recalculated after import. Nothing in the notebook is sent anywhere, and saving a result here is not reporting it to anyone.