such events to be seen convincingly with amateur equipment, the same
being true for the brighter moons of Uranus and Neptune.
If the body that’s passing in front of another is too small to occult the more
distant one, the event is called a transit. Transits are quite common in the
Jovian system where the four Galilean moons may be seen to pass in front
of Jupiter’s disc. Such events are common for Io, Europa and Ganymede but
less so for the slower moving and more distant Callisto.
Transits of the main Saturnian moons may also be seen as long as the
planet’s tilt is such that the Earth is more or less in its equatorial plane. Such
events are much harder to see than those which occur in the Jovian system,
requiring a large telescope to view. Having orbits which are smaller than the
Earth's, the inner planets Mercury and Venus may be seen to transit the Sun.
Transits of Mercury occur 13 or 14 times per century while those of Venus
are much rarer, occurring in pairs separated by 8 years but with a long
wait of 105.5 or 121.5 years between pairs. The whole 243-year cycle then
repeats. The next transit of Venus occurs in 2117 with another in 2125.
Transits of Mercury can only occur in May and November. The nearest
transits of Mercury to the time of writing this book will occur on May 9th
2016, November 11th 2019 and November 13th 2032.Sun-Moon Interactions
The Sun and the Moon have similar apparent diameters in the sky, but the
elliptical orbit of the Moon around the Earth coupled with the elliptical orbit of
the Earth around the Sun means that the apparent size of both bodies does
vary slightly. If the apparent size of the Moon is smaller than normal and the
size of the Sun is larger, it’s possible for the Moon to appear to pass in front
of the Sun’s disc without totally covering it. If you’re standing in the right place
on Earth, the outer edge of the Moon appears to be totally surrounded by the
edge of the Sun. This transit event is also known as an annular eclipse of the
Sun. The appearance of the Moon’s dark silhouette central within the Sun’sOccultations of stars by a major planet do also happen but, again the
planet’s relatively small disc size means such events aren’t that common
and occultations of bright stars by planets are very rare. The magnitude
+3.3 star Theta Ophiuchi will be occulted by Mercury on 4 December 2015.
Magnitude +2.9 Pi Sagittarii gets the same treatment by Venus on 17
February 2035, while brilliant Regulus in Leo is also occulted by Venus on 1
October 2044.
A more common planetary occultation occurs when a major planet with
moons appears to hide one of its moons. This can be seen to occur with
Mars, Jupiter, Saturn, Uranus and Neptune, although only Jupiter shows
events which are fairly common and easily seen with a small telescope.
Occultation of Saturn’s brighter moons by the planet can only occur when
Saturn’s tilt angle is fairly edge-on to Earth, something that occurs at
approximately 15 year intervals. The two moons of Mars are too faint for56Astronomer Book