GCSE Science Assessed Practical
Physics - Astronomy
Rotation of the Sun
The Sun rotates in
space. Your task is to find the
relationship between the angular velocity and rotational period of the Sun’s
surface. You should use scientific knowledge
to explain how measurements of features on the Sun’s surface can be used to
calculate the angular velocity of the surface and the period of rotation.
The Earth’s surface is made
of rock. As the planet rotates, the
whole surface moves together. Wherever
you are on the Earth’s surface, it takes 23 hours an 56 seconds to make a
complete rotation relative to the stars (sidereal day). The angular velocity is the same for any
point on the planet
i.e. approximately 360o / 24hrs = 15o/hr
If this were not so, the
Earth’s surface would be ripped apart as the planet rotated. However the speed at which the surface moves
is not the same over the globe. The
equator of the Earth is 40000km in circumference, so the equatorial velocity
is;
40000 / 23.9 = 1670km/hr
This will decrease with
higher latitudes.
However the Sun does not
have a solid surface; it is made of gas which can flow. The angular velocity of the Sun does not
have to be the same at different latitudes.
Does the surface of the Sun
rotate as a whole, like the Earth, or is the rate different near the poles
compared to the equator? You could
conduct preliminary experiments. For
example you could experiment with liquids in containers or wet paint on
spheres.
Write a plan for your investigation. Think about other variables that could affect
your results. Remember that as you make
your observations you are on a planet that is orbiting the Sun.
Use your own observations
using an optical instrument (ASK YOUR TEACHER ABOUT SAFE METHODS) or using
remote observatories via the internet, to gather measurements. You could use a mixture of the two. Decide on the number of observations needed
to make reliable conclusions. Give
details about the instruments used.
Show your results in a clear
form with all units labelled. You may
find a spreadsheet useful in analysing you results. Display your analysis in a suitable
graphical form.
Make conclusions backed up
by scientific ideas.
Evaluate you results. Look for any anomalies in your data. Say how more accurate results could be
obtained.
Measuring Angular Distances on a
Spherical Body
The distances that appear on
a sphere are not linear. As can be seen
in the grid above, the equal segments of latitude and longitude appear to get
smaller towards the limbs of the sphere.
Each segment represents 1/12 of the sphere’s diameter.
To measure distance on the
Sun, the grid can be superimposed onto images of the Sun’s disk in programs
such as Photoshop.
.
In this image the pair of
sunspots in the lower left quadrant are separated by
approximately half of a 15o segment – 7.5o or 1/24th of
the diameter of the Sun at that latitude.
If the left-hand sunspot took 55 hours to reach 0o longitude (i.e.
move 30 o) the rotational period will be (360/30)55 = 660 hrs or
27.5 days. The greater the proportion
of the sun’s surface over which a feature is tracked, the more accurate the
results should be.