Astronomy Principles and Practice Fourth Edition.pdf

Chapter 12
Geocentric planetary phenomena
12.1 Introduction
On July 21st, 1969, an event unique in the history of mankind took place. For the first time ever, a
human being observed the heavens from the surface of a celestial object other than the Earth. When
Neil Armstrong stepped onto the Moon’s surface, he gave mankind a shift in perspective that until then
had had to be imagined, for all previous direct views of the Sun, Moon, planets and stars had been
made from one position, namely the Earth.
Observations of celestial objects from the Earth are usually referred to as geocentric observations
although, strictly speaking, it should be remembered that any particular observation is made from a
point on the Earth’s surface and has to be ‘corrected’ or ‘reduced’ to the Earth’s centre before it can be
said to be truly geocentric.
The Earth’s changing position round the Sun complicates the apparent behaviour of celestial
objects (their geocentric behaviour); they have observed movements that in part are due simply to
the velocity of the observer’s vehicle—the Earth—just as objects in a landscape viewed from a moving
car appear to have movements they do not, in fact, possess.
Paradoxically, the nearest and the farthest celestial objects are the least affected by the Earth’s
movement, though for different reasons.
The nearest natural object of any size, the Moon, accompanies the Earth round the Sun, moving
about the Earth in an orbit that is roughly elliptical. The apparent movement of the Moon against
the stellar background is, therefore, almost entirely due to its orbital motion; the component of shift
caused by the observer viewing the Moon from the rotating surface of the Earth is small. If we,
therefore, observe the Moon’s sidereal position at a given time each night, we find that the Moon
moves eastwards against the stellar background by approximately 13 ◦ per 24 hours. Its sidereal period
of revolution about the Earth is found to be about 27 1 3 days.
The farthest celestial objects are the galaxies. They are at distances so great compared to the
diameter of the Earth’s orbit that the Earth’s yearly journey about the Sun cannot affect their observed
positions on the celestial sphere. Only 120 000 of the nearest stars in our own galaxy are close
enough to have had their annual parallactic shifts measured accurately by the Hipparcos satellite (see
section 10.7.3).
The Sun’s apparent sidereal movement is due principally to the Earth’s orbital movement. We
have seen that the Sun appears to move in a great circle—the ecliptic—at a rate of about 1 degree per
day, returning to any particular stellar position in one year. Because the orbit is an ellipse, equal angles
are not swept out in equal times by the line joining the centres of Earth and Sun.
In the case of the planets, their geocentric movements are the most complicated of all, being
compounded of their own orbital movements and the Earth’s. The ancients observed that these objects
not only ‘wandered’ with respect to the celestial background but occasionally appeared to change their
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