Astronomy Question: Apparent Size and Magnitude of the Sun from Other Planets?

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Doug asks,

Dear Astronomer,
What is the size and brightness of the Sun as viewed from the different planets in the Solar System?

Excellent question, Doug!

As the distance increases between a planet and our Sun, not only do planets receive less energy, but the apparent size deceases.

Standing on the surface of Mercury would yield an impressive sight, while standing on Eris, the sun would be much fainter.

Keep reading for a detailed comparison of the Sun’s apparent size through the Solar System.

For starters, here’s a little math. We can calculate the apparent size of the sun with the following equation:

                     180°    Measured diameter
Apparent diameter =  ---  x  ----------------
                      π      Distance from object

Example using Earth:

180°    1,380,000 km
---  x  ----------------  = .527 degrees.  
 π      150,000,000 km

Interestingly enough, the Sun’s apparent diameter as seen from Earth is nearly that of our Moon.
(Hence why the Sun is blotted out during an eclipse.)

The resulting apparent diameter is in degrees, so it’s not exactly intuitive. Let’s put this into a “real world” perspective.

The human thumb is about 2cm wide. Extend your arm out, stick out your thumb and you’ll cover about 2° of sky.
Your closed fist covers about 10°.

As seen from Earth, the Sun’s apparent diameter of .527° means your thumb is about four times wider than the Sun when held at arms length.

The formula above is pretty easy to use, so forgive me for not calculating all of them.
Here’s a few examples though to give you an idea of how distance affects the Sun’s Apparent diameter.

Mercury ~ 1.36° Which is just over half the width of your extended thumb if you were standing on Mercury.
Mars ~ .35° A bit over half the apparent diameter as seen from Earth (See above image for a real-life example)
Titan (Saturn’s Largest moon) ~ .055° About one-tenth the apparent diameter as seen from Earth! This is only from a distance of 10AU – not even half way to Pluto!
Neptune ~ .017° Neptune is thirty times more distant from the Sun than Earth.

As seen from Eris, past Pluto in the Kuiper Belt, our Sun would look like a very bright star.

Despite these relatively small apparent diameters, the apparent magnitude of our Sun is still quite impressive at great distances.
To answer the second part of your question, here’s a short table of apparent magnitudes.

Mercury ~ -29
Earth ~ -27
Mars ~ -25
Titan ~ -22
Neptune ~ -19
Eris ~ -17 (Still several orders of magnitude brighter than the full Moon!)

Keep in mind with apparent magnitude the scale is logarithmic. If you want to learn more about the math behind apparent magnitudes, check out this link:
http://hs.riverdale.k12.or.us/~dthompso/math/starmag.html

I hope this helps shed some light on how distance affects the apparent size and magnitude of our Sun.

Thanks for sending in your question!

Ray Sanders is a Sci-Fi geek, astronomer and blogger. Currently researching variable stars at Arizona State University, he writes for Universe Today, The Planetary Society blog, and his own blog, Dear Astronomer

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