8.9 Practical 7: The Speed of Light
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8.9 Practical 7: The Speed of Light
In 1676, the Danish astronomer Ole Rømer noticed that the eclipses of Io, in front
and behind Jupiter, lagged behind what was predicted by orbital mechanics and that
the lag seemed to be dependent on the distance of the Earth from the planet. Rømer
realised that this implied that the speed of light was finite, and he managed to measure
it at close to current estimates. With modern high-speed cameras and increasing
computing power, it is possible for you to follow in Ole Rømer’s footsteps.
8.9.1 Aims
In this practical, you will image a solar system object using a high-speed astronomical
camera or webcam with the goal of identifying the moment in time at which an
occultation occurs. By looking at the difference between the predicted moment that
it should occur (without accounting for the speed of light) and the actual moment of
occurrence, you will then be able to calculate the speed of light.
8.9.2 Planning
This project needs careful planning, as you will identify an occultation or transit that
will take place during your observing time. If Jupiter is visible, then the transit of Io
is the best candidate, as such transits are common and obvious. If not, you could use
a planetary occultation of a star. Try to avoid using the Moon, since although lunar
occultations of bright stars are common (planets less so), the Moon is very close and
will introduce large errors into your results.
You will need to use a piece of planetarium software to find the uncorrected time
of the occultation or transit. Stellarium is excellent for this. Just remember to turn
off the option to simulate light speed found in the view window. You will also need
to find the distance between the Earth and the solar system object at the point of the
occultation or transit. Again Stellarium can do this for you.
You are likely to generate a large video file, so make sure you have some way
of getting this off the camera control computer. You will also need to carefully
measure the moment the transit is observed, so ensure that you have an excellent
well-calibrated timepiece.
129
8.9 Practical 7: The Speed of Light
In 1676, the Danish astronomer Ole Rømer noticed that the eclipses of Io, in front
and behind Jupiter, lagged behind what was predicted by orbital mechanics and that
the lag seemed to be dependent on the distance of the Earth from the planet. Rømer
realised that this implied that the speed of light was finite, and he managed to measure
it at close to current estimates. With modern high-speed cameras and increasing
computing power, it is possible for you to follow in Ole Rømer’s footsteps.
8.9.1 Aims
In this practical, you will image a solar system object using a high-speed astronomical
camera or webcam with the goal of identifying the moment in time at which an
occultation occurs. By looking at the difference between the predicted moment that
it should occur (without accounting for the speed of light) and the actual moment of
occurrence, you will then be able to calculate the speed of light.
8.9.2 Planning
This project needs careful planning, as you will identify an occultation or transit that
will take place during your observing time. If Jupiter is visible, then the transit of Io
is the best candidate, as such transits are common and obvious. If not, you could use
a planetary occultation of a star. Try to avoid using the Moon, since although lunar
occultations of bright stars are common (planets less so), the Moon is very close and
will introduce large errors into your results.
You will need to use a piece of planetarium software to find the uncorrected time
of the occultation or transit. Stellarium is excellent for this. Just remember to turn
off the option to simulate light speed found in the view window. You will also need
to find the distance between the Earth and the solar system object at the point of the
occultation or transit. Again Stellarium can do this for you.
You are likely to generate a large video file, so make sure you have some way
of getting this off the camera control computer. You will also need to carefully
measure the moment the transit is observed, so ensure that you have an excellent
well-calibrated timepiece.