40 votes
Accepted

Is the Sun visible from Proxima Centauri to human eyes?

Well, there's two things we'll need for this: apparent magnitude (the brightness that an object appears to have) and absolute magnitude (the actual brightness an object has). Both of these scales are ...
Sir Cumference's user avatar
26 votes

Which is brighter, Mars as seen from Earth, or Earth as seen from Mars?

We can use the expression that is commonly used to estimate the apparent magnitude of a planet or asteroid in the Solar System: $$\boxed{m=5 \log \frac{1329}{d \cdot \sqrt p}+5 \log (D_s \cdot D)-2.5\...
Albert's user avatar
  • 1,720
23 votes
Accepted

How bright will Alpha Centauri A be from Earth when it becomes a red giant?

First of all, by the time Alpha Centauri A becomes a red giant, it will no longer be this close to the Sun due to the orbit of the stars around the galaxy so it probably wouldn't be visible. But let's ...
WarpPrime's user avatar
  • 6,613
20 votes

Why are nearby stars like Proxima Centauri and Barnard's star not visible to the naked eye?

John's answer is correct. For a few more details: Stars brightness works out to roughly the 4th power of their relative mass. This falls off a bit for very large stars, but for smaller stars it's ...
userLTK's user avatar
  • 23.9k
20 votes
Accepted

Could Neptune be viewed with the naked eye from Uranus?

According to https://arxiv.org/pdf/1808.01973.pdf, the magnitude of Neptune follows the relationship (formula 17, page 25): $ V = 5 \log_{10} (rd) - 7.00 + 7.944 \times 10^{-3} α + 9.617 \times 10^{-5}...
Pierre Paquette's user avatar
17 votes
Accepted

Which is brighter, Mars as seen from Earth, or Earth as seen from Mars?

At it's brightest, Earth is a rather impressive magnitude -2.5 when viewed from Mars1 (the maximum brightness depends on how favourable the elongation is, but it isn't usually brighter than -2) At the ...
James K's user avatar
  • 116k
14 votes

Have there been successful attempts at spotting Neptune with the naked eye?

Well, having seen Neptune and identifying Neptune are two totally different things. Let's tackle this one step at a time. Your link says: From these observations it would appear that, if seen upon ...
Sir Cumference's user avatar
14 votes

Could Neptune be viewed with the naked eye from Uranus?

Supplementary answer supporting @PierrePaquette thorough and well-source answer: I tried the nice new JPL Horizons interface and fired up Excel which I haven't used in a long time. For years 1800 to ...
uhoh's user avatar
  • 31.4k
13 votes

Which is brighter, Mars as seen from Earth, or Earth as seen from Mars?

Without much math, the answer is relatively simple when we look at two facts: Earth is bigger than Mars. Size of the cross-section (or rather illuminated area) matters for brightness. The path of the ...
planetmaker's user avatar
  • 17.5k
11 votes
Accepted

Determining the extinction by counting stars?

The general idea is that the more extinction there is, the less you'll be able to see stars. So there is a direct relation between the decrease in flux observed for any individual star and the number ...
usernumber's user avatar
  • 17.4k
10 votes
Accepted

What stars are brighter than any more distant stars?

It is a fairly short list, due to the uncertainty in determining distance to stars. Name Magnitude Distance Note Sun -27 1AU No star closer, no star brighter Sirus -1.46 8.6 The only star brighter ...
James K's user avatar
  • 116k
9 votes
Accepted

As viewed from Mars, what are Jupiter's and Saturn's maximum brightness in apparent magnitude?

The inverse-fourth-power law you're referring to is valid for light emitted from a source, reflected non-specularly — i.e. in all directions — from a reflector, and detected by the original emitter. ...
pela's user avatar
  • 37k
9 votes
Accepted

Brightest Stars (by its Apparent Magnitude) List beyond 300

The Yale Bright Star Catalogue lists stars brighter than about magnitude 6.5. It is available as a compressed text file documented in the readme file I've also made a google sheets version sorted by ...
James K's user avatar
  • 116k
9 votes

Could Neptune be viewed with the naked eye from Uranus?

The brightness of a Solar System object, seen in reflected light, depends on how far it is from the Sun, $d_s$, and how far away it is from the observer, $d_o$, (and the angles between them). Both ...
ProfRob's user avatar
  • 146k
9 votes
Accepted

Adhara ultraviolet magnitude

Those making the claims should define exactly what they mean. U-band magnitudes are well defined. The Johnson U-band covers from about 300-400 nm with a peak at 360 nm. The peak wavelength would be ...
ProfRob's user avatar
  • 146k
8 votes
Accepted

Snowball Earth brightness

Earth is already pretty bright due to cloud cover, with a typical albedo of .3-.35 -- that is, it reflects about a third of the visible light that hits it. That means it couldn't get more than about ...
jeffB's user avatar
  • 380
7 votes
Accepted

Sun's apparent magnitude at 2.5 AU?

What is the apparent magnitude of the Sun at that distance? Fortunately, there's a formula known as the Distance Modulus which is defined to calculate precisely this information. The equation is ...
zephyr's user avatar
  • 15k
7 votes

A what distance could we see the Sun?

I agree with your naked eye figures, but the sensitivity of JWST has been over-estimated - it could "see" the Sun at 4 million light years. The Sun has an absolute visual magnitude of about ...
ProfRob's user avatar
  • 146k
6 votes

How bright is the full Earth during the lunar midnight?

Ask Ethan: How bright is the Earth as seen from the Moon? has some detailed explanations, including discussions of lunar eclipses as seen from the moon. It doesn't include magnitude calculations, but ...
nealmcb's user avatar
  • 412
6 votes

Is the Sun visible from Proxima Centauri to human eyes?

Alpha Centauri A and B happen to be rather similar to Sol, and their absolute magnitudes are 4.38 and 5.71 respectively (Wikipedia). Add them together and you get absolute magnitude 4.10 (the scale ...
Anton Sherwood's user avatar
6 votes
Accepted

Why is there "modulus" word in the "distance modulus" term?

According to Oxford Dictionary of English, the word "modulus" is the diminutive of the Latin modus, meaning measure (modus, in turn, comes from Proto-Indo-European mod-os, Nocentini & Parenti 2010....
pela's user avatar
  • 37k
6 votes
Accepted

Total apparent magnitude of eclipsing binary system

Without information about stellar radii, I think it's reasonable to assume $R_A \approx R_B$. Then your equation becomes $$ m_p - m_s = -2.5 \log \frac{F_A}{F_B} $$ and you can compute $k$ and the ...
Mike G's user avatar
  • 18.2k
6 votes
Accepted

What is the "lunar irradiance" received by the Earth from the full Moon?

You can work in magnitudes, but the magnitude scale is logarithmic. Instead you can use luminosity. The moon has a luminosity that is 400,000 times less than the sun, so the maximum theoretical &...
James K's user avatar
  • 116k
6 votes
Accepted

If Planet Nine indeed exists, how large a impact event there should be to be seen from Earth?

We can compare the brightness of a comet hitting planet 9 with the 1994 impact of Shoemaker-Levy 9 / SL9 on Jupiter. It was seen by Hubble, and Galileo which was orbiting Jupiter at the time. View ...
bandybabboon's user avatar
  • 4,188
6 votes

How long does lunar opposition surge last? Are there measurements of the full Moon getting suddenly brighter?

The lunar opposition surge has been well studied, likely because we can study it in detail, we have surface samples, and so it serves as a baseline for other bodies in the solar system (as it does for ...
Stuart Robbins's user avatar
6 votes

Is subtracting one apparent magnitude from another ever practical?

Two stars in a binary system are at the same distance from Earth. If they have similar spectral types then the difference in their magnitudes tells us the ratio of their luminosities. $$\Delta m = m_1 ...
ProfRob's user avatar
  • 146k
6 votes

What is the smallest (brightest) magnitude that the JWST can safely observe?

The Bright object observation by JWST is still being studied. The known problems that arises with Bright object observation is image persistence. Although it is said that James Webb space Telescope ...
Kavin Ishwaran's user avatar
6 votes
Accepted

Is Uranus the farthest object in Solar System that you can see with naked eyes?

Yes, apart from maybe Neptune, which is within the "extreme naked eye limit", there are no other objects further than Uranus that have an apparent magnitude above +8. See this list: https://...
Rabbi Kaii's user avatar
6 votes

Sky surface brightness vs magnitude limit visibility

The human eye has an angular resolution of about 1 arcmin. This means that the light coming from point like stars would be visible if it significantly exceeds the luminance of the sky over an area of ...
Ritesh Singh's user avatar

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