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Conjunctions in Astronomy

This Sunday 26th April, the three-day-old Moon will be in conjunction with the brightest planet, Venus. Conjunctions are a brilliant example of astronomical phenomena you can observe from your back garden with your own two eyes! They occur when astronomical objects appear close to each other in the sky. When these objects are particularly bright, you don’t even need any specialist equipment to view a conjunction. So, let’s dive in to conjunctions in astronomy!

What is a conjunction?

Technically, a conjunction is defined as an event where two or more celestial objects have the same right ascension or ecliptic longitude. These are both east-west coordinates for two different coordinate systems used to locate objects in the sky: the celestial coordinate system and the ecliptic coordinate system. However, conjunction is more commonly used to describe any event where celestial bodies appear close to each other.

Some conjunctions happen more often than others. For example, the Moon is often in conjunction with the other planets of the Solar System, because it moves very quickly across the sky. It is far more rare for distant planets, such as Uranus and Neptune, to be in conjunction with each other, because their great distance mean they move very slowly. Don’t forget, the speed with which something moves across the sky is not the same as its true speed relative to you – the distance makes a big difference!

How to observe

It’s easy to observe most conjunctions: you simply need an unobscured view and a clear night! It all depends on the objects you are trying to see. The Moon and Venus are both bright objects, so they will be clear to the naked eye. Pay attention to the difference in brightness between the objects as well. If one object is much brighter than the other, its light may drown the other out! This is especially important for conjunctions involving the Moon. The Moon is very finnicky – its phases mean that the light it gives off can be very different depending on when in its cycle you are observing. This Sunday, the Moon is in the relatively faint phase of waxing crescent, so it won’t be able to hide Venus from you!

Whilst brighter conjunctions like this can be seen with the naked eye, binoculars work wonders. Often overlooked, a decent pair of binoculars will greatly enhance your view, and allow you to see some of the fainter conjunctions as well. And if you’re lucky enough to have access to a telescope, even better! This Sunday though, the Moon and Venus will be too far apart to view with binoculars – but all the better to see with your own eyes!

The Moon & Venus

Now for the specifics – when and where you can see this Sunday’s conjunction. The 2 bodies will become visible at about 20:40, and set around 00:35, so you’ll have plenty of time to spot them. Look for the constellation of Taurus in the west, and you’ll see the crescent Moon passing just south of Venus. Not sure how to find Venus? Well, you now know whereabouts to look, so just look for the big bright star – it may look like the others but it is in fact another planet! The conjunction will be fairly low in the sky, so make sure you don’t have anything too tall in your way.

Labelled constellation diagram of Orion and Taurus.
The Moon and Venus in the constellation of Taurus. 21:00, Woking.

Now that you know all about conjunctions, get out there and see one for yourself! Check out our Sky Notes page for a calendar provided by In The Sky, where you can keep track of any conjunctions as they happen.

Sources: In The Sky, Royal Museums Greenwich