Chance alignments of objects in the Solar System can introduce a
dramatic element into astronomy, with tremendous potential to
create exciting images. Here we look at conjunctions, occultations,
transits and eclipses, events which involve one or more bodies
appearing to interact with or obscure one another. Such events
only occur at certain times, something which adds another level of
excitement to the proceedings.EVENT DRIVEN ASTRONOMY
the already stunning view. Conjunctions between the Moon and the other
planets can also occur, and those between Jupiter, Mercury and, to a lesser
extent, Saturn, can also be impressive sights to behold. There’s no real
scientific worth to a conjunction, this being an astronomical event to just
savour and enjoy.
Conjunctions between the Moon and planets are quite common because
all of these bodies travel across the sky close to the ecliptic – the apparent
path of the Sun against the background stars. This is a great circle around
the sky which has the Earth at its centre. The ecliptic can also be thought of
as defining the plane of the Solar System, and the orbits of the major planets
and the Moon tend to lie close to this plane. Consequently, when you see a
major planet or the Moon in the sky, it will be lying quite close to the ecliptic.
From this description it’s possible to work out that the major planets can
also be in conjunction with one another. However, having a slower apparent
daily motion against the background stars, conjunctions between planets are
far less common than those between the Moon and the planets. The Moon’s
a fast mover and will pass all of the planets at least once every month. Catch
two bright planets, such as Jupiter and Venus, close together in a relatively
dark sky, and the sight can be very impressive indeed. When three or more
planets or two planets and the Moon appear close to one another in the sky,
this is known by the infrequently used term "a massing".
The Moon has a tangible size in the sky: an apparent diameter of
around 0.5 degrees. During its monthly travels against the background
constellations, the Moon’s disc may sometimes pass between us and
a planet, hiding the planet from view. Such an event is known as an
occultation, or more specifically, a lunar occultation. Despite the Moon’s
apparent size in the sky, lunar occultations of planets aren’t that common.
Lunar occultations of a planet are fascinating to watch through a
telescope. As the Moon catches up with the planet, the edge of the Moon
can be seen to cross the planet’s comparatively tiny disc before covering
it completely. This is what’s known as an occultation disappearance
event. After a period of time which depends on where the planet happens
to be passing behind the Moon’s disc, the planet will reappear from
behind the trailing edge of the Moon. This is known as an occultation
reappearance event.
A special kind of occultation occurs when you see the edge of the Moon
partially clip the planet’s disc but never completely cover it; an event known
as a grazing lunar occultation. As an aside, this sort of event can also happen
between planets, although the small size of a planet’s disc compared
with the relatively slow speed that planets move relative to one another
means that such events are quite rare. The next such event will occur on 22An apparent close approach of one body to another can be quite an
impressive sight. One of the most dramatic is that which occurs when the
planet Venus and the crescent Moon appear close together in the sky.
Although just being close together does not fulfil the strictest definition
of the term, it’s common to describe the apparent pairing of one or more
bodies in the sky as a conjunction. The formal definition states that they
must have the equivalent of the same celestial longitude for it to be a true
conjunction, but close together seems an acceptable definition for most of
these situations.
As Venus can never venture too far from the Sun, any conjunction that
happens between it and the Moon must occur when the Moon is in its
crescent phase; that’s an old crescent in the morning sky or a young crescent
in the evening sky. This means that the event normally takes place during
twilight with a blue gradient sky background adding an extra dimension to[1] Venus peeks out from behind the Moon at the end of a lunar occultation of the
planet1Astronomer Book