Measure the diameter of the Sun

Summary

In this activity you will measure how fast the Sun moves to caclulate how big the Sun appears in the sky. All you need are some household items and about 20 minutes on a sunny day.

Goals

Carry out your own experiment and use your results to calculate the diameter of our closest star; the Sun.

Learning Objectives

Students will understand the fixed speed of the Sun as it appears to travel across our sky.
Students will learn how to measure the movement of the Sun across the sky and use their results to calculate the angular and physical size of the Sun.
Students will develop an understanding of the scale of the Sun compared to the Earth and the Moon.

Planning

For this activity you will need the following apparatus:

A small mirror
A piece of thick paper to cover your mirror
A tripod or a lump of Plasticine or playdough
Something to use as a screen (a big sheet of white paper with a cardboard backing will do)
A pencil or marker pen which will write on the screen
Masking/electrical/duct tape
A stop watch

Background

The Sun is the centre of our Solar System, provided all of the light and heat needed to support all living things on our planet. Our lives are dominated by the movement of the Earth relative to the Sun, we use these movements to measure time: one day is the length of a full rotation of the Earth on its own axis and one year is a full orbit of the Earth around the Sun. However, many people do not possess an accurate idea of the scale of the Sun relative to the Earth.

It takes 24 hours for the Sun to travel 360 degrees, all the way around the sky and return to the same position it was in the previous day. The speed the Sun travels at is: 

360 degrees / 24hours = 360 deg/(24x60) minutes = 0.25 deg per minute = 0.00416 degrees per second or 1/240 degs per second

Instructions

Introduction

The Sun moves across the sky at a constant rate because of the rotation of the Earth. By measuring how fast the Sun moves you can work out how big the Sun appears in the sky. All you need are some household items and about 20 minutes on a sunny day.

Full Description

Creaing a Solar Reflector

Cut a small hole in the centre of the piece of thick paper.

Tape the paper with the hole to the front of your mirror. Ensure that the hole is over your mirror, covering up nearly all of the reflective surface.

Tape the mirror to the tripod, so that the mirror faces up.

If you don’t have a tripod, put plasticine/playdough on the floor in an L-shape (as demonstrated in the photograph below) and embed the block on it so that the mirror is at about a 40-degree angle (facing the Sun).

This is your solar reflector. Stand it up and make sure it is stable and will not move during the experiment. If it does move you will have to start all over again!screen up and make sure it cannot move during the experiment - if it does move you will have to start all over again.

The assembled solar reflector.

Using your Solar Reflector

Angle the solar reflector so that Sun is reflected onto the screen.
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).
Enter your results into the table below and take the mean average duration of all your timings (in seconds).

Time 1	Time 2	Time 3	Average Duration

Next Steps

Discussion and Results

What you have been doing is 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 the Sun travels at is: 

360 degrees / 24hours = 360 deg/(24x60) minutes = 0.25 deg per minute = 0.00416 degrees per second or 1/240 degs per second

1. Now, 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

2. You can now 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 × (π/180) × 149 598 000 km = _______________ km

Congratulations, you have measured and calculated the diameter of the Sun in miles/kilometers!

Peer Reviewed version

This activity has been peer-reviewed by independent experts in astronomy and education. It is available on astroEDU:

Measure the Solar Diameter

Activity