How Good is your Night Sky?

An easy-to-use walk around guide.

By Garry P. Dalrymple

The visibility of the stars and the planets is a restriction placed on all of us who love the sky. Wasteful and unnecessary lighting is a particular concern to all Sydney City Skywatchers (except our radio astronomers?) but it is the price we pay for living in a big city. As I walk my dog by night, I frequently look up at the stars, and going from main streets to backstreets and across deserted parks, what you can see often varies surprisingly, if you care to go out and explore, you may find that there are some ‘darkish’ sites surprisingly close to where you live.

How it works – The southern constellations of Crucis (The Southern Cross) and Musca (the fly) are compact and easily found. They are circumpolar stars, which means that (cloud permitting) they will be out there all night slowly turning around the ‘bolt’ that holds the sky on, the South Celestial Pole, and these stars are always out of the way from the city glare.

What you do is to look for these two constellations and count as many stars as you can see clearly, and then count how many more stars you can see, sort of, but not quite clearly, and these you would score as a ‘half’, so that on a clear night you may count – four and a half Crucis and one and a half Musca stars – which would give you a naked eye seeing limit of between magnitudes 3 and 3.5, but more usefully, on the same night you can go and repeat this process for several locations and perhaps find a nearby viewing spot that is a little bit darker than most corners of your backyard or your balcony.

You do not need to write anything down, or to make any calculations, but if you get into the habit of looking up and to the south most nights, you will soon discover that the ‘seeing’ does vary over the phases of the moon, as well as between ‘Good Air’ and ‘Bad Air’ days, which might help you with your asthma or other respiratory health issues, to be able to predict good or bad days, the night before.

Below is a star map of these two constellations taken from Stellarium, as well as a corresponding table showing the stars in order of brightness. You can download Stellarium or other similar apps to your phone or laptop. You can also find constellation maps on Wikipedia. If you have any difficulty doing this, simply ask a ‘younger person’. I mean, the only reason why we brought them into the world was for future tech. support.

The Southern Cross and Musca as orientated in the southern sky in late March (adapted from https://stellarium-web.org/).

Letter	Brightness (Magnitude)	Name	Constellation
α Alpha	0.8	Acrux	Southern Cross
β Beta	1.25	Mimosa	Southern Cross
γ Gamma	1.63	Gacrux	Southern Cross
α Alpha	2.69		Musca
δ Delta	2.8	Imai	Southern Cross
β Beta	3.05		Musca
ε Epsilon	3.5	Ginan	Southern Cross
δ Delta	3.61		Musca
λ Lambda	3.63		Musca
γ Gamma	3.87		Musca
ε Epsilon	4.05		Musca
μ Mu	4.75		Musca

This article grew out of a discussion between Sydney City Skywatchers members about the recent lack of clear nights. One of our members has developed a method using gravity to set up and orient a telescope to the stars, almost instantly, but he needs clear skies in order to verify that his method works as accurately as many existing ‘find several stars and adjust’ methods.

Help protect our dark skies.

Our view of the night sky is literally disappearing before our eyes. A 2023 study published in the journal Science found that light pollution is increasing by up to 10% per year. You can help reverse this trend by supporting your local dark sky association and by learning to improve the lighting around your home. In Australia see australasiandarkskyalliance.org and internationally darksky.org.