Academy, Saturday 18th December 1875
The Opposition of Mars, 1877.—The Astronomer Royal has given
a chart of the path of Mars in 1877, with neighbouring stars down to the ninth
magnitude taken from Bessel's Zones, the object being to facilitate
observations of the parallax of Mars in this important opposition, when the
planet approaches more closely than usual to the earth, owing to the position
of its eccentric orbit.
Although it is hoped
that the sun’s parallax will be determined with considerable accuracy by the
late transit of Venus, astronomers cannot afford to throw away the opportunity
which the planet Mars offers of getting an independent, and possibly equally
trustworthy, determination, more especially when it is remembered how important
a part the observations of Mars in 1861 (the last opposition which was
favourable for this purpose ) played in correcting the results obtained from
the transit of Venus in 1769, For observations of Mars no special expeditions
will be necessary, all that is required being to measure the apparent distances
of the planet from neighbouring pairs of stars at northern and southern
Observatories, and also at the same Observatory, when the planet is rising and
when it is setting. The latter class of observation enables an observer to
determine the sun’s distance by measures made entirely at his own Observatory,
the rotation of the earth providing him with a base line from east to west.
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