- Cut a small hole (something between a pinhole and 5mm diameter hole) in the centre of the piece of thick paper.
- Tape this piece of paper to the front of your mirror, so that you can only see the mirror through the small hole you made.
- Attach the mirror to the tripod, so that the paper faces upward.
- If you don’t have a tripod, put the plasticine/playdough on the floor and embed the block on it so that the mirror is at about a 40 degree angle (facing the Sun).
- Stand your screen up and make sure it cannot move during the experiment: if it does move, you will have to start all over again.
- Angle the mirror so the Sun is reflected onto the screen.
- The projected image of the Sun must be circular, so angle your mirror and screen until it is.
- Trace around the image of the Sun on the screen.
- Start your stopwatch.
- Wait until the Sun has moved to just outside the circle you drew.
- Note the time on the stopwatch and reset.
- Repeat steps 2–5 a few times (repeat 3–4 times for more accurate results).
- Take the mean average of all your timings (in seconds).
Discussion and Results:
In this activity, you have been measuring how long it takes the Sun to move a distance equal to its own diameter across the sky. The Sun will take 24 hours to travel 360 degrees all the way around the sky and return to the same position it was in on the previous day. The speed it travels at is:
= 360 deg/(24x60) minutes
= 0.25 deg per minute
= 0.00416 degrees per second or 1/240 degs per second
Calculate the size of the Sun as an angle:
Average duration (in seconds) × 1/240 (degrees per second) = _ degrees
Congratulations! You have calculated the angular size of the Sun.
Calculate the physical size of the Sun
You can use your value for the angular size of the Sun to calculate the physical size of the Sun.
Your number for angular size converted into radians × distance from Earth to the Sun = size of the Sun
Angular size × (pi/180) × 92 955 887.6 miles = _ miles OR
Angular size × (pi/180) × 149 598 000 km = _km
Congratulations! You have measured and calculated the diameter of the Sun in miles/kilometers!