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45 votes

Why is the Sun's density less than the inner planets?

The sun isn't the same density all the way through. According to MSFC's solar interior page, the core density at the centre of the sun is a whopping 150,000 kg/m$^3$. Surrounding it the radiative ...
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39 votes
Accepted

Why doesn’t the Sun fill the sky on Mercury?

From where we stand on Earth, Mercury is pretty small about 13 arcseconds across at most. The sun, by comparison is about 1800 arcseconds across, so if you are to see Mercury as a disc, you need to ...
  • 96.2k
32 votes

Why doesn’t the Sun fill the sky on Mercury?

[The real answer is in @James K's answer (it's to do with the field of view of your top image being tiny, but the second one is quite wide). This is to translate the situation into one that might be ...
29 votes

Why is the Sun's density less than the inner planets?

Fusion inside of a star affects the sun's density (which does not happen with a planet). It produces an outward pressure that balances against the attraction of gravity, thereby reducing the density ...
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22 votes
Accepted

Does anything orbit the Sun faster than Mercury?

The recently discovered asteroids 2019 LF6 and 2020 AV2, each taking 151 days to orbit the Sun, have the shortest periods currently listed in the JPL Small Body Database. Vulcanoids are difficult to ...
  • 16.9k
22 votes
Accepted

What rotational speed would Mercury need to have to achieve a temperature comfortable for humans

There is no rotation speed which can achieve that - globally. For a local analysis see below. The global equilibrium surface temperature in the absent of any atmospheric greenhouse effect and with an ...
  • 13.3k
21 votes

Why is the Sun's density less than the inner planets?

The density of matter depends not only on its composition, but also on temperature and pressure. It's not meaningful to say that substance A is denser than substance B without specifying the ...
18 votes

Why doesn’t the Sun fill the sky on Mercury?

I can make a similar image using Stellarium, where the Sun seems huge compared to Saturn. Yet when on the surface of Saturn, the Sun seems much smaller in the sky than it does from Earth. Or in other ...
  • 16.9k
16 votes

Does anything orbit the Sun faster than Mercury?

There is a problem with very high velocity dust particles orbiting the Sun at distances much closer than Mercury with masses of micrograms and below. These particles can do damage to spacecraft that ...
  • 30.6k
14 votes

What are the "weather" (and other) implications of long "days" and short "years" on Mercury?

Mercury has no atmosphere, so it experiences direct radiative heating and cooling. The path of the sun in the sky is a slow motion lasting many Earth days and is due as much to the orbit of Mercury (...
  • 96.2k
13 votes

Why is the Sun's density less than the inner planets?

I'd say the most important answer is because the volume of stars is counted differently than for (inner) planets.For the former, most of the gas surrounding the dense core is counted. The latter don't ...
  • 239
13 votes

Why doesn’t the Sun fill the sky on Mercury?

For intuitive understanding: if Mercury were a lot bigger, but in the same orbit, the view of the sun from the surface of the planet (2nd photo) would be the same, but from our perspective (1st photo),...
  • 131
10 votes

Are there ever any simultaneous transits of both Mercury and Venus as seen from the Earth?

As others have calculated, there are no predicted double transits. Since Venus transits for about 12 hours each hundred years (roughly), Venus is in transit for about 1/100000 of the time. Thus ...
  • 96.2k
10 votes
Accepted

Why is Mercury's Density So Low?

The actual density depends on the mineralogy, we don't have a crystalline iron core and silicon crust. You do have a lot of oxygen available, too when you look at the overall elementary abundance. So ...
  • 13.3k
9 votes
Accepted

Can we find rocks from Venus or Mercury on Earth?

You can think of it in terms of Hohmann transfer orbits, which define the minimum $\Delta v$ that needs to be applied to bring something from one orbital radius to another orbital radius when orbiting ...
  • 124k
8 votes

Could Venus or Mercury have a moon that we haven't detected?

It's unlikely that either Mercury or Venus could have moons to begin with. Both of these planets are pretty close to the Sun — and in general, this prevents moons from finding stable orbits. If a ...
8 votes

Path of Mercury and general relativity

And we know that many of astronomers know that mercury orbit does not precess ... Mercury's orbit does precess, by a good amount. The greatest amount is explained by Newtonian mechanics. Venus, ...
  • 29.4k
8 votes

Does anything orbit the Sun faster than Mercury?

A hypothetical asteroid that orbited the sun inside the orbit of Mercury would be called a vulcanoid. No vulcanoids have so far been discovered, although there is a population of asteroids that cross ...
  • 647
8 votes
Accepted

Could Mercury be a some kind of Chthonian?

Surely not a gas giant... Mercury is a rock of 0.05 $\rm m_{\oplus}$, such a mass could never grow into a gas giant (For detail see Piso & Youdin (2014), fig. 6, where you can see that below a ...
7 votes
Accepted

Is it possible that Mercury was originally the moon of Venus after a giant impact?

This was originally going to be a comment, but it ran too long, so I'm making it an answer. Some models argue that the scenario of a satellite of Venus escaping like this is unlikely. Alemi & ...
  • 34.4k
7 votes

Shall we say now that Pluto is "larger than Mercury" ?

The implication of the question is that this extra 1000 miles should be added to Pluto's radius. The answer is no. For all of the solid planets, it's that solid surface (or solid+liquid surface in the ...
  • 29.4k
7 votes
Accepted

Can Mercury's core "boil over" if it gets to close to the Sun?

I was reading about how mercurys core makes up most of the volume of the planet. Im guessing this is both because of the small size and the distance to the sun. There are at least two hypotheses ...
  • 587
7 votes

After what time interval do the closest approaches of Mercury to the Earth repeat?

@Glorfindel's explanation is very clear. I wondered about how big of an effect "everything else" would have, but I couldn't figure out how to add an image to a comment. Here is a graph from ...
7 votes

Does anything orbit the Sun faster than Mercury?

2022 update. The asteroid 2021 PH27 has an orbital period of 114.6 days, setting a new record for any non-Mercury solar system object.
6 votes
Accepted

Is Mercury's core liquid?

It's liquid. As detailed here, To figure out whether Mercury's core was liquid or solid, a team of scientists led by Jean-Luc Margot at Cornell University measured small twists in the planet's ...
  • 34.4k
6 votes
Accepted

How large was Mercury before it shrunk?

If we look at the planet's cores and I'm going to ignore liquid vs solid and focus on size overall. Mars: Core estimated 1,794 +/- 65 km radius. The planet is 3,390 km radius. About 53% of the ...
  • 23.1k
6 votes
Accepted

Why does in-the-sky.org say that Mercury is not visible from Taipei these days?

The logic that In-The-Sky.org uses to decide whether events are observable or not doesn't currently work very well for Mercury and Venus. I have some plans to fix this in the long term, but always ...

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