Binocular Astronomy – Brightness & Magnitude

The following article on understanding the Brightness of objects in our night sky is part of a series of Binocular Astronomy guides written for Best Binocular Reviews by the astronomer Philip Pugh:

Understanding Brightness & Magnitude

Astronomers use a measurement called magnitude to determine the brightness of stars and other objects in the sky. It is expressed as a decimal number.

Objects that are very bright have negative magnitudes.

Some objects have fixed magnitudes, as they do not change much in brightness and others vary in magnitude. For example, planets vary in magnitude, as their distance from us varies considerably. Some stars expand and contract and their brightness varies over days, weeks or years.

Binoculars Star Magnitude
Just how bright objects like galaxies, stars, planets and their moons appear in our skies at night is important and especially important in binocular astronomy as it not only helps you locate them but can help you decide what size and power of binoculars you need.

The Sun

The Sun has a magnitude of -26.7, making it the brightest object in the sky, by far. It is so bright that if you look at it through binoculars you will not only hurt your eyes but cause permanent damage to them. You should only use approved solar filters.

View of the Sun with binoculars & filters

The Sun with binoculars & filters

The Moon

The Moon varies in magnitude, due to its phase and its distance from us. A full ‘supermoon’ where the full phase coincides with a close approach to us delivers its greatest brightness of -12.6. Although a bright moon can cause discomfort to your eyes, it does not cause permanent damage.

These figures suggest that the Moon is half as bright as the Sun, but this is misleading! Magnitude operates on a logarithmic scale and a magnitude difference of 1 between two objects results in a brightness difference of 2.512 times. To make the math simpler, a magnitude difference of 5 results in a brightness difference of exactly 100. The difference in brightness between the Sun and ‘supermoon’ is about 390 000 times!

The Moon

At the fainter end, stars and other objects with a magnitude of 6.0 are visible in a dark sky using neither binoculars nor a telescope. In practice, most of us live in light-polluted skies and there is a thin haze about most nights.

In extreme urban environments, I have experienced skies where the limiting magnitude (the faintest star that you can see) is about 2.0. From my suburban home, the limiting magnitude is about 4.5 most nights.

Larger objects, such as the Andromeda Galaxy are more difficult to see than their magnitude suggests. This is because their magnitude is a measure of the total brightness of the object.

As a rough guide, larger objects are as difficult to see as stars that are two magnitudes fainter. Therefore galaxies; star clusters and nebulae are referred to as ‘faint fuzzies’. Fortunately, even small binoculars can reveal many of these, such as the Andromeda Galaxy (however better views will be attained using larger Astro binoculars). I will provide details on it (and other objects) in forthcoming articles.

Four of the planets can outshine any of the stars in the sky. The table below lists the maximum magnitude of the major planets, excluding Earth:

Planet Maximum Magnitude
Venus -4.6
Mars -3.0
Jupiter -2.7
Mercury -2.0
Saturn -0.5
Uranus 5.7
Neptune 7.7

Uranus is theoretically visible without binoculars but, in practice, I have never achieved it. Fortunately, binoculars with an aperture (objective lens diameter) of 30mm can show both Uranus and Neptune, if you know where to look. Using a search engine, such as Google, shows where each planet is, relative to the stars, and what time it rises and sets.

Canis Major, featuring Sirius

Canis Major, featuring Sirius

Canis Major, featuring Sirius

The brightest star in the sky is Sirius (the Dog Star), which has a magnitude of -1.46. It is visible on winter nights in the northern hemisphere and is nearly overhead from the most populated places south of the equator. It is in the constellation of Canis Major, the great dog. The second brightest star is Canopus, which has a magnitude of -0.72, about half as bright as Sirius.

Alpha Centauri has a magnitude of -0.27 and is a double star, but it needs a telescope to resolve it into its two components. Neither Alpha Centauri nor Canopus are visible from northern temperate latitudes. The final star with a negative magnitude is Arcturus at -0.04, which is in the northern part of the sky and is well-placed during the northern hemisphere spring.

Increasing limiting magnitude with Binoculars

Binoculars extend the limiting magnitude from any location. If the exit pupil (aperture /objective lens size divided by magnification) is roughly the same as the width of your eyes’ pupils, the limiting magnitude is determined by the table below.

Binocular Aperture (mm) Increase in Limiting Magnitude (approx.)
20 +2.9
25 +3.3
30 +3.7
35 +4.1
40 +4.4
45 +4.7
50 +5.0
55 +5.4
60 +5.7
65 +6.2
70 +6.5
75 +6.8

In practice, these figures reduce under conditions that are not totally dark, as the width of your eyes’ pupils are less than the exit pupil of your binoculars.

Even without knowledge of how to navigate the night sky, you notice that you can see many more stars when you casually browse the night sky with binoculars. If you can see the Milky Way, then that is a great place to start. You will notice that the band of white becomes hundreds of individual stars. As you try successively larger binoculars, the number of stars you can see increases even more.

The stars that are visible in the night sky give you a rather misleading impression. Any stars you can see are usually closer or naturally brighter.

Comparing Real Brightness
To compare the real brightness of stars, we measure how bright they would look if placed at 32.6 light years from us. This is called the absolute magnitude.

At this distance, our Sun would become a faint star, with a magnitude of +4.8. Sirius would “dim” to magnitude +1.3, at a similar distance.

As Arcturus is about 29 light years from us, its brightness would stay about the same, but its magnitude would be slightly positive, instead of slightly negative.

The brightest stars would be Rigel (in Orion) and Deneb (in Cygnus), both about magnitude -7.1, making them clearly visible in daylight!

In most skies, the limiting magnitude in daylight is about -3.0 to magnitude -3.5. This makes Venus visible if it is not too close to the Sun in the sky. Jupiter and Mars have been seen in daylight but not by me.

Further Reading

For more in this series and more in general on using binoculars for astronomy, take a look at these guides:

Guide to Choosing the Best Binoculars for Astronomy
Viewing the Moon with Binoculars by Philip Pugh
Viewing Planets Through Binoculars by Philip Pugh
Guide to Using Binoculars for Astronomy by Philip Pugh
Best 10x50 Binoculars for Astronomy

About Philip Pugh:

Philip describes himself as an “uninteresting person who does interesting things”. He had his first magazine article published in 1980 and has published four books in Patrick Moore’s “Practical Astronomy” series including those shown below.

Like nearly all writers, he has a “day job” and has spent most of his working life as a trainer or technical author. These days, astronomy is his only active interest outside of work and family. He is best known for his interest in the Sun and one of his many activities is to monitor activity using binoculars (with suitable filters). He is also a keen photographer and takes quality photographs using budget equipment.

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