Finding Arcturus
UNDERSTANDING THE NIGHT SKY Complex maths isn’t necessary with William Thomson’s expert guidance
Having moved aboard Luna to renovate her for a circumnavigation, my favourite part of our new home was being so close to the night sky. In particular, I would watch Arcturus race westwards, rising up towards the south then swooping down to the west. This journey happened a little earlier every day, owing to the star being due south two hours earlier every month. In this feature we will explore why this happens, learn how to predict which stars will be out on any night of the year, and share techniques to navigate by them.
Let’s start with the basics. The stars appear to travel westwards because the earth is rotating on its axis. Once a day the earth makes a 360º orbit, which equates to 15º apparent westward movement per hour. For the navigator, a crucial part in this orbit is when your meridian on earth lines up with a star’s meridian on the celestial sphere, a moment called ‘culmination’. This is when the celestial body is highest in the sky and the time of day it happens depends on the position of the sun in the celestial sphere at that time of year – it moves 30º around the sphere every month on a journey called the ‘ecliptic’, shown by the yellow in the diagram.
When the sun is on the same side of the ecliptic as a star, both culminate at the same time – noon – and you cannot see the star because the sun’s light is too bright. But six months later, when the sun is on the opposite side of the sphere, the star is highest in the sky at midnight and suitable for navigation. When working out which stars will culminate in the night, you want to look for those on the opposite side of the celestial sphere to the sun at that time of year, which can be done by observing sidereal hour angle (S.H.A), the name for celestial longitude. As a general rule, stars with a S.H.A of 000-090 are summer stars, 090-180 are spring stars, 180-270 are winter stars and 270-000 are autumn stars. This can be a lot to remember, but by aligning a star’s S.H.A on your Star Calculator (download it for free at www. tidalcompass.com/store using the code #YM at checkout) you can see the time of culmination every week of the year.
With an S.H.A of 146º, Arcturus frequents the night sky around spring and summer. But what use is this to the navigator? Firstly, you can use the methods we explored last month to work out latitude at the time of culmination. Secondly, because Arcturus culminates due south from the mid-northern latitudes, you can use this as a natural compass. Working on the principle that stars move 15º per hour, you can use the time of culmination as an ‘anchor point’, subtracting or adding 15º to 180º south for every hour before or after culmination (this only works on stars that move ‘horizontally’ through the sky). For example, if it is 2300 at the end of June and your Star Calculator shows that culmination was at 2000, you can deduce that Arcturus will have moved around 45º to the west since it was due south three hours ago, so it will be south-west. You can quickly glance up at the night sky and use this to get your bearings – but only in the spring! In the autumn, you will need to find a star with a different S.H.A; thankfully there are 58 navigational stars to choose from, carefully selected to cover their own patch of the night sky.