The background stars in the night sky are a long distance from Earth. The Moon, on the other hand, is
a relatively close neighbour of the Earth. This gives observers on the Earth the
potential to see parallax in 'action'. If, in Diagrams 1 and 2, the red ball is replaced with the
Moon and the number strip with the stars in the night sky, it should be clear that if
two (or more) observers can observe the Moon at exactly the same time from different positions on the
Earth's surface, then it should be possible to see parallax.
The distance from the Earth to the Moon is approximately 400,000km (250,000 miles).
The Earth's diameter is approximately 12,756km (7,926 miles). This would be the
maximum distance possible between Observer 1 and Observer 2. In practice,
geographical (and meteorological!) effects will mean that observers are separated
by less than the possible maximum distance.
In addition, using the real figures for the Moon's distance and Earth's diameter,
it is clear that the diagrams above are very much exaggerated.
Diagram 3 shows the Earth-Moon system more-or-less to scale. Note that the lines drawn from
opposite sides of the Earth to the Moon have the maximum possible separation (12,756km/7,926 miles) and are
therefore showing the maximum possible parallax angle.