BBC Sky at Night Magazine
CONJUNCTIONS: a stargazer’s guide
The Moon, planets and stars will be coming together to produce some amazing night-sky sights over the coming months. Paul Money reveals what to look out for
Conjunction: what a funny-sounding
word it is, yet in the field of
astronomy this phenomenon can give us some wondrous night-sky sights, ranging from naked-eye views,
through to binoculars and even telescopic viewing.
Generally speaking a ‘conjunction’ is the name given to two or more celestial objects close together in the night sky. The most commonly observed conjunctions involve the Moon, often as a crescent in the evening or morning sky, along with any of the bright planets – Venus, Mercury, Mars, Jupiter or Saturn. You can also see conjunctions between the Moon and bright stars or even between the planets themselves, so there is quite a range of possible combinations. Some involve more than two objects, such as when two planets are in conjunction and are joined by the Moon. There are also times when incredibly close conjunctions set two objects in the same telescope field of view, or in really special cases, show Venus or Mercury transit across the face of the Sun.
You may already have come across the term in
astronomy guides, yet if we went by its strictest
definition then some events called conjunctions would probably not qualify. To be precise: a conjunction is a line-up of at least two celestial objects in the sky relatively close together that share the same Right Ascension (RA) or ecliptic longitude in the sky. RA is the equivalent of longitude on Earth but projected onto the celestial sphere. The ecliptic is the plane of Earth’s orbit and appears to us as the apparent path of the Sun across the sky. Ecliptic longitude is measured along the ecliptic eastwards from the spring equinox. Even within astronomy there are different meanings of the word conjunction. When a planet, either outer or inner, lies on the other side of the Sun to Earth it is said to be at ‘superior conjunction’. When an inner planet lies between Earth and the Sun it is at ‘inferior conjunction’.
Often a conjunction will occur during daytime or when the objects are below the horizon, and this is where the definition becomes more relaxed. If the objects are very bright, such as a crescent Moon and Venus, then daylight viewing can be possible, but if the objects have set below the horizon they won’t be visible. So conjunction can be applied in quite a loose context, to refer to objects that are viewable above the horizon in twilight or at night, even if they are not, at that point, at the exact moment of conjunction. If the objects are at their closest, then this is known as an ‘appulse’: the minimum separation between the
two bodies that occurs just before or after the true
Conjunctions really capture our attention, which makes them ideal targets for public stargazing
events, or for inspiring young astronomers and
newcomers to look up at the night sky. They are also
easy to capture with a smartphone camera, giving
more people the chance to preserve the moment and share with friends or on social media.
Over the following pages we’ll look at some fascinating conjunctions coming up over the next few months between planets, the Moon and stars. Keep a look out for these beguiling events, and make sure you follow our monthly Sky Guide (see page 43) in every
issue for more information about how you can