A
determination of this quantity and the radius of the Earth, allows one to
compute the Astronomical Unit – The mean distance of Earth and the Sun. With
modern measurements we know the value of Solar Parallax as 8.794148 seconds of
arc.
The
most significant seminal paper outlining the possibility of measuring the
Astronomical Unit, using the Transit of Venus observations from two widely
separated geographical locations, was that of Halley, published in the
Philosophical Transactions (1716).
A simple way of understanding the method
indicated by Halley, may be summarized as follows. First,
some asides.
The
discs of the planets Mercury or Venus, as seen from Earth, are much smaller than
that of the moon, of course. Therefore they make no more than a small black dot when they
move in front of the face of the Sun. It takes more than a couple of hours,
usually, for this small dot Mercury or Venus to cross the face of the Sun. With
every transit, depending on the geometry involved, this dot may walk a different
path across the face of the Sun, but, one thing will remain unchanged – its
overall direction of motion. In the Solar System, almost every motion is
anticlockwise – if one works out what this means in terms of Earth’s Spin,
Sun’s movement in the Sky and the motion of Moon and the planets Mercury and
Venus, one will see that, invariably, Ingress – the point of entry in front of
the Solar disk, for Mercury or Venus, is always from the East and exiting on the
western edge, and the reverse for the Moon.
The first ingredient, needed from the
observations, are the contact timings.
There are four important points of time during the transit that astronomers would like to measure. These are called 1st, 2nd, 3rd, and 4th contact. 1st contact, which is very hard to observe, is the point in time when the Planet first touches, apparently, the disk of the sun. 2nd contact is the time at which Venus is fully engulfed within the sun's disk and is internally tangent to the disk. 3rd contact is when the disk of Venus just begins to leave the face of the sun, and 4th contact is when the disk of Venus has just completely left the sun's disk and is externally tangent to it. While inside the disk of the Sun, Venus moves across at a rate of about 4 arc minutes /hour. The greatest transit happens when the centers of Venus and Sun would appear to have the minimum distance as seen from the center of the Earth. The diagram below clarifies these concepts –
