Measuring The Speed Of Light | Science Experiment

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In this experiment, we will be measuring the speed of light by using the concept of Parallax. What we have done basically is measured the distance of Jupiter from the earth at opposition. Then we derived equations for calculating this distance at other times. We observed and recorded the times of numerous LO (Jupiter’s innermost moon) eclipses and used these to find a time lag.

Finally, by coupling the distance changes and the time lag, we measured the speed of light.


Measuring the speed of light by determining the distance to Jupiter using the concept of Parallax.


1. The speed of light in a vacuum is 3 X 10^8 meters/second.

2. Parallax is a concept that is being used in astronomy to describe the difference in distance between two celestial bodies by looking through two different lines of sight.



1. Telescope

2. Star Map

3. Pen

4. Paper

6. Stopwatch

7. Clock


Step 1: Find the distance at opposition using the parallax using a telescope.

Step 2: Derive an equation to measure the distance to the Jupiter

Step 3: Record the timing of various eclipses. And observe them carefully.

Step 4: Use the obtained timing to measure the time lag.

Step 5: Calculate the speed of light by using the distance changed and time lag.

Step 6: Note down your observation in the paper.


1. We observed that 4.2 Astronomical Unit (actual 4.4 AU) was the distance to Jupiter at opposition.

2. The change in the distance of Jupiter was 0.0486 AU (actual 0.0482).

3. This gave the speed of light as 3.83 X 10^8 m/sec (actual 299,729 km/sec)


1. We obtained the measurement of parallax with only a 5% error.

2. The change in the distance had only a 1% error.

3. Instead of the measured distance is accurate, the overall speed of light was very sensitive to small timing errors.

4. The result that we obtained for the speed of light was too high. It was 30% faster than the accepted value of the speed of light.

5. To get good results, it is important to obtain extreme accuracy in your observation. And our project obtained a much higher accuracy.


1. Record your observation precisely.

2. Telescope should be working properly.


In this experiment, measuring the speed of light, we determined the speed of light by observing the distance to Jupiter.

Viva Questions With Answers

Q.1 What was the aim of your experiment?

ANS. We aimed to measure the speed of light by estimating the distance to Jupiter and observing various eclipses.

Q.2 State the parallax formula.

ANS. The formula for parallax is d= 1/p. Where

d is the distance to the star and

p is the parallax angle that is measured in arc-second.

Q.3 What speed of light did you obtain in this experiment?

ANS. It was 3.83 X 10^8 m/sec.


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