Size of the Sun
Project for Elementary Astronomy
In this project you will make a simple measurement of the Sun using a homemade
pinhole projection device, then use this measurement and the known distance to the
Sun to calculate the Sun's diameter. The
key to getting good results is making careful
WARNING: At no point in this project do you need to look
directly at the Sun. Directly looking at the Sun can cause serious
and permanent damage to your eyes.
To conduct this observing project, follow this procedure:
To complete the project, include the following in your analysis:
- Construct a solar pinhole projector like
the one shown in the figure above. You will need some
heavy cardboard or cardstock, some aluminum foil, and a sheet
of white paper on a clipboard to use as a projection screen. The larger the
piece holding the pinholed aluminum foil the better; it will cast a
darker shadow on your drawing surface, making it easier to sketch
the Sun. The hole in the cardboard, which is covered by the aluminum
foil, should be only an inch or two across.
- You may try several different pinhole
sizes. Small holes, made with a single pin-prick, work for short projections (a meter or so),
while larger pinholes work for larger projections (images have been
projected over 20 meters!). In general, smaller pinholes are
better. Start small; you can always make a small pinhole bigger!
- Make your observations on a clear day when
the Sun is reasonably high in the sky. Rigidly support the pinhole device on a
ladder, chair back, camera tripod or post (for example). The
observations will work best if the screens are all perpendicular to
the direction to the Sun (especially the drawing screen).
- Use the pinhole to cast an image of the
Sun on your screen. Make sure that the projected image is many times
larger than the pinhole itself; if it isn't, move the screen farther back.
Outline the image with a pencil, then measure its diameter and its distance from
Repeat this procedure at different distances (and with different pinhole sizes
if you wish) to collect at least 5 measurements.
- One way to make accurate measurements is to
pre-draw precise circles of different sizes on your drawing screen,
then adjust the distance to the pinhole so the Sun's image exactly
fills each circle. This reduces your field measurements to only
the distance between the screens.
- If you have trouble seeing the projected image of the Sun
at a great enough distance, try setting up your viewing screen so it is mostly shaded
from the sky. It is actually best to make your measurements indoors, with the sun
shining in a south-facing window.
- If possible, take a photo of your observational setup showing both the pinhole device
and the projection screen. Include this photo in your report.
- The estimate of the solar diameter is
based on the idea of similar triangles; in this case the
two triangles are the one formed by the solar image with the
pinhole, and the one formed by the Sun and the pinhole. The
analysis of these similar triangles says that the ratios of the
diameters to distances are equal:
- Look up the known distance to the Sun,
and use the above formula to calculate the diameter of the Sun for
each of your measurements. Be careful to use the same units in
all cases. I recommend you work in meters. Round off your
answers (for the sun's diameter) to the nearest million meters or so.
- Average your results and compare them to
the official value of the diameter of the Sun. Calculate the percentage
difference between your result and the official value. Cite your source(s)
for the distance and for the official value of the diameter.
- Your report should also explain how the distance
to the Sun (which you used in your calculation) is measured.
Unfortunately, this measurement is much more difficult so you won't
be able to make the measurement yourself. Be sure to cite the whatever
sources you use for this information.
- Please attach your pinhole device (perhaps cut down to
a reasonable size) and your projection screen (with sun images drawn) to your
report. If you enlarged your pinhole in the course of your observations, be sure
to indicate which pinhole sizes were used for which observations.
- Be sure to follow the general instructions applicable
to all projects.