“Can I see Jupiter with my naked eyes in the night sky?” and other similar questions…

Introducing the scale of “apparent magnitude” in Astronomy

The celestial meet-up of Jupiter, Venus and the Moon. Photo Credits: Twitter/@Astrofotografen. Source: News18.com

You might have already come across some beautiful shots of the planetary conjunction of Venus, Jupiter and the Moon from across the world this last week — if you haven’t, step out in the evening and search for the Moon in the westward sky. You will see Venus and Jupiter trotting behind the Moon.

Now this is a rare celestial event at display with their closest alignment on March 1, 2023 — and one of the reasons why this is so special and relatable for everyone is because you don’t need any equipments to see this confluence. But we know that’s not always the case. In fact, even in this case, we know that we can see Jupiter with our naked eyes — but we definitely cannot see Jupiter’s moons without a telescope.

So, what decides which objects in the night sky I can see with my unaided eyes vs which are the ones for which I would need binoculars and equipments?

There are multiple factors at play, but one scale to combine them all that’s used by amateur astronomers and professionals alike since historical times is the scale of “Apparent Magnitude”.

What is Apparent Magnitude?

Let’s think of the basics. If we look up at the sky, objects that are brighter can be seen by us easily but we cannot see the ones which are dimmer. That’s essentially what Apparent Magnitude captures.

It is the relative brightness of an object in the night sky as observed from the Earth. It is called “Apparent” and not “Absolute” because this measure of brightness is relative to Earth, and it depends on a combination of multiple factors like -

1. Distance of the astronomical object from the Earth -> the more distance, dimmer is the object.
2. Size of the object -> Bigger objects are easily observable and will seem brighter in the sky.
3. Actual energy/radiation emitted by the object per diameter -> Any astronomical object emits the same amount of energy in all directions, but the more energy that can reach us in the visible light wavelength, the brighter it will appear to us in the night sky.

At what apparent magnitude can I see objects with my naked eyes?

The Apparent Magnitude scale is a logarithmic scale where lower is the magnitude, brighter is the object. This has led to some confusion about the scale because —

“negative or lower apparent magnitude = more brightness” !

The scale of Apparent Magnitude in astronomy. Picture Credits: EarthSky

Here, difference in 1 magnitu6de = 2.512 times brighter object w.r.t. to the dimmer object, according to the Pogson relation below (built by N.R Pogson in 1856 on top of the scale suggested by Hipparchus during Greek civilization.)

The Pogson relation to calculate apparent magnitude. Source: Spiff.rit.edu. Here, m1, m2 are the apparent magnitudes of two astronomical objects, while I1 and I2 are their respective brightness.

According to this scale,

The unaided eye can see objects upto a magnitude of +6.0,

and the binoculars can be used to see objects upto a magnitude of 10, beyond which we will need telescopes. Also, for some perspective, according to SkyLive.com,

the planet Jupiter is shining these days at a magnitude of -2.14 , while Venus is at a magnitude of -3.94.

A chart representing the apparent magnitude of well-known objects in the night sky and telescope limits. Chart created by author with images from the internet, not made to scale. Inspired by Year10Constellation and University of Central Florida

Do different places on Earth have different apparent magnitude?

Yes! At the end of the day, apparent magnitude is how bright an object appears in the sky to you on Earth. So this can vary from observer to observer and can also vary based on the amount of light pollution in any particular region. But it is an excellent thumb-of-rule to get a generic idea of how bright/dark an astronomical object will be in a relatively dark area.

Do objects change their apparent magnitude from time to time?

Yes! Nothing’s constant in the Universe, so there are objects which:

• Change their apparent magnitude in a relatively short span of time due to a change in their relative distance from Earth (we see this very often with comets and planets)
• Or they change it over a very long period of time, i.e., thousands of years, because they are undergoing a different part of their lifecycle ( for instance, it is predicted that Betelguese, the red giant, will shine at a magnitude of -12.40 from 0.58 today when it becomes a supernova!

That’s all for the introduction of apparent magnitude! Now the next time you read any article about the magnitude of a star or an astronomical object in the sky related to any celestial event, you can use the scale above as your yardstick to know if you can catch the event with your unaided eyes.

Apparent magnitude is also a very handy tool to find the relative brightness of any unknown object as an amateur astronomer! All you have to do is to know the apparent magnitude of a standard object like Sirius the star — and then calculate the magnitude of the unknown object relative to this standard. The formula has already been discussed above; you can check out more details about the method here if this sounds interesting!

Obviously, this is just based on what we see on Earth, so what scale do astronomers use with telescopes that are in space?

The answer is that they need to calculate brightness with respect to some standard in space called Absolute Magnitude. Stay tuned to learn more about Absolute magnitude in the next Space Nugget and how it helps us create a map of the Universe!

This article was edited by Vagisha Bhatia and Atotmyr.