# Astronomy Physics A Level Question HELP!

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#1
The moon Titan has an angular diameter of 4.4 mrad as seen from the surface
of Saturn. The Sun has an angular diameter of 9.3 mrad as seen from the
surface of the Earth. Which of the following eclipses cannot be seen from
the surface of Saturn?

A. A lunar eclipse of Titan by Saturn.
B. A partial solar eclipse due to Titan.
C. A total solar eclipse due to Titan.
D. An annular solar eclipse due to Titan

I'm not sure how to approach this question and also am unsure what the difference is between a partial, total and annular solar eclipse. The correct answer is C, can anyone help?
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2 years ago
#2
(Original post by ryan__lai__)
The moon Titan has an angular diameter of 4.4 mrad as seen from the surface
of Saturn. The Sun has an angular diameter of 9.3 mrad as seen from the
surface of the Earth. Which of the following eclipses cannot be seen from
the surface of Saturn?

A. A lunar eclipse of Titan by Saturn.
B. A partial solar eclipse due to Titan.
C. A total solar eclipse due to Titan.
D. An annular solar eclipse due to Titan

I'm not sure how to approach this question and also am unsure what the difference is between a partial, total and annular solar eclipse. The correct answer is C, can anyone help?
Sure
Total solar eclipse is when the moon blocks all of the surface of the sun... this requires that the angular diameter of the moon is greater than or equal to the angular diameter of the sun
Partial solar eclipse is when the moon blocks part of the surface of the sun
Annular solar eclipse is when the moon blocks the central part of the sun leaving a ring of sun surface visible around the edge of the moon.(annular just means ring shaped)... the requirement for an annular eclipse is that the angular diameter of the moon is less than the angular diameter of the sun.

The question gives you the angular diameter of Titan seen from Saturn and the angular diameter of the sun seen from earth... you need to know the angular diameter of the sun seen from saturn to decide which eclipses are possible based on the conditions I gave above.
This requires another piece of information - the orbital radius of saturn in AU (which is approximately 9)

since the earth orbits at a radius of 1AU the angular diameter of the sun as seen from saturn is approximately 1/9 th the angular diameter of the sun as seen from earth (this is a small angle approximation... appropriate in this case because these are all small angles)

the angular diameter of the sun seen from Saturn is therefore 9.3/9 mrad = 1.0... which unfortunately contradicts the 'correct' answer - titan with angular diameter 4.4 mrad is very able to totally eclipse the sun with an angular diameter of 1.0 mrad - the type of eclippse which would be impossible is the annular.
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