Library · Experiment
Track sunspots to time the Sun's spin
Project the Sun safely onto card, sketch its spots over a couple of weeks, and work out how fast our star rotates.
- When
- Mid Dec – Jan · Into space
- Time
- 10 minutes on each clear day, over ~2 weeks
- Cost
- £0 with binoculars you own
- Difficulty
- 2 of 3
Part of The Road Map (Into space) · Measuring the Solar System (Measure the Sun)
The question
The Sun is a ball of gas, not a solid. How long does it take to turn once — and can I measure it from my garden?
Learn just enough first
- Read what sunspots are and why there are more of them in some years than others
- Learn the projection method below before starting
Kit
- Binoculars on a tripod, with one lens covered by its cap
- White card for a screen, plus a second card with a hole to shade it
- Pencil, and a circle traced on paper to match the projected Sun
Safety: Never look at the Sun with your eyes, binoculars, or a phone camera through binoculars. Only ever look at the image projected on the card. Never leave the binoculars pointed at the Sun unattended, and keep children and pets clear.
My prediction
How many days do I expect for one full turn? Will spots near the middle seem to move faster than spots near the edge?
Method
- Point the binoculars at the Sun by watching their shadow — when the shadow is smallest, the Sun's image lands on the card behind.
- Focus until the edge of the Sun is sharp, then trace the circle and mark each spot.
- Repeat on every clear day, at about the same time, for around two weeks.
- Pick one spot group and track how far it moves across the disc each day.
Data
| Date | Time | Spot group | Position on disc (notes) |
|---|---|---|---|
Result
A spot takes roughly half a rotation to cross the visible disc. Seen from Earth, one turn takes about 27 days near the Sun's equator. How close is my estimate?
Explain it to a friend
Sunspots are cooler, darker patches carried round as the Sun turns. Draw them each day and you can watch the Sun spin — slightly faster at its equator than near its poles, because it isn't solid.
New questions
- Do spots at different latitudes move at different speeds?
- Why do spots appear to squash up near the edge of the disc?