THE SOLAR SYSTEM
The tides are largely
raised by the Moon, but
the Sun also has an effect.
When they act together, the
tides are strong (spring
tides). When they act at
right angles, the tides
are weak (neap tides).about it. In fact it has proved to be basically the same
as the region we have always known, though the surface
features are arranged in a somewhat different manner.
The Moon has a crust, a mantle and a core. There is a
loose upper layer, termed the regolith, from 1 to 20 metres
(3 to 65 feet) deep; below comes a layer of shattered
bedrock about 1 kilometre (0.6 miles) thick, and then a
layer of more solid rock going down to about 25 kilome-
tres (15 miles). Next comes the mantle, and finally the
core, which is metal-rich and is probably between 1000
and 1500 kilometres (600 to 930 miles) in diameter. The
core is hot enough to be molten, though the central tem-
perature is much less than that of the Earth.
The Moon’s low escape velocity means that it has
been unable to retain much atmosphere. A trace remains,
and was detected by instruments taken to the Moon by
Apollo 17 in 1972; the main constituents are helium (due
to the solar wind) and argon (seeping out from below the
crust). The atmosphere seems to be in the form of a colli-
sionless gas, and the total weight of the entire atmosphere
can be no more than about 30 tons. The density is of the
order of around 10^14 that of the Earth’s.The Half Moon, Last Quarter
(7). The rays are less striking;
shadows inside the large
craters are increasing.
The Old Moon (8) occurs
just before the New, seen
in the dawn sky. Earthshine
may often be seen.Distance from Earth, centre to centre:
max. (apogee) 406,697 km (252.681 miles)
mean 384,400 km (238,828 miles)
min. (perigee) 356,410 km (221,438 miles)
Orbital period 27.321661 days
Axial rotation period 27.321661 days
Synodic period (interval between
successive New Moons) 29d 12h 44m 3s
Mean orbital velocity 3680 km/h (2286 miles/h)
Orbital inclination 5° 9’
Apparent diameter: max. 33’ 31”
mean 31’ 6”
min. 29’ 22”
Density, water = 1 3.34
Mass, Earth = 1 0.012
Volume, Earth = 1 0.020
Escape velocity 2.38 km/s (1.48 miles/s)
Surface gravity, Earth = 1 0.165
Albedo 0.07
Mean magnitude at Full: 12.7
Diameter 3476.6 km (2160 miles)LUNAR DATAB Atl of Univ Phil'03stp 31/3/03 4:04 pm Page 43