Alison Mudie of Dunedin asks :-
A recent article stated that astronauts on the moon could blot out the earth using their thumb at arms length. Is this so?
Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.
From the earth, the full moon is half a degree across. A little finger at arms's length is about a degree across. So most adults should easily be able to hide the moon behind their little finger-nail.
The sun and moon subtend nearly the same angle at the earth, the moon can almost completely eclipse the Sun when it gets between us. Hence the sun too, can be blocked behind a little finger at arms length but the glare from dusty air around the sun makes that harder on the eye.
The earth is 3.7 times wider than the moon. Seen from the moon, it is 1.9 degrees across. In round figures the earth looks four times wider than the moon. A thumbnail is about 1.5 degrees across at arms length; a full thumb-width about 1.8 degrees across. So an astronaut on the moon would easily hide the earth behind a gloved thumb.
Because the moon's orbit is slightly elliptical its distance from us varies. So the apparent size of the moon changes, but not by much. At closest the moon is 33.4 minutes of arc across, or 0.56 degree wide. At furthest it is 29.4 minutes of arc, 0.49 degree wide. The 12 per cent difference is hardly noticeable by eye.
Then there is the 'moon illusion': the rising or setting moon always looks bigger than does the moon when it is high in the sky. This is entirely due to image processing in the brain. In fact the rotation of the earth takes us 2 per cent closer to the moon when it is high in the sky compared to when it is rising or setting.