Below is my attempt at solving this question. The question was:

Fig. 21-3 (6th edition) shows our Sun as a Red Giant with a diameter of 1AU. At this time, what will the angular extent (how many degrees across the sky) of the Suns disk be as viewed from Earth?

There are a few simplifying aspects to the solution I give below. Firstly I assume that the Sun has a well defined edge. I also assume that the distance from Earth to the centre of the Sun is exactly 1AU. Also I assume that when all of this happens the Earth is still in its present orbit. ... (and I assume that an observer is still able to view the Solar disk!!). In my diagram E is for Earth; C is for centre of Sun; S is for surface of Sun.

Some elementary trigonometry will help here. There is a good web site at www-istp.gsfc.nasa.gov/stargaze/Strig3.htm that discusses sine, cosine and tan functions.

In our (simplified) case the angle between the lines ES and SC is a right angle or 90 degrees. It has to be because the line of sight along ES must just graze the surface of the Sun; by definition this has to be perpendicular to the line (SC) from the surface to the centre at this point. Hence we have a right-angled triangle and the law of sines can be utilised. I derive the angle, alpha to be 30 degrees, but the total Solar disk will span 2x alpha or 60 degrees (or 120 Full Moon diameters!).

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