Hot answers tagged

84

When looking at the first image, you need to take into account perspective distortion in images with a narrow field-of-view. This is the same effect that makes people look closer together than they actually are when photographed through zoom lenses (which is useful to bear in mind when reading the news these days!). Mercury is not as close to the Sun as the ...


45

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 zone is around 20,000 - 200 kg/m$^3$ (already less dense than water). Eventually at the edge is the convective zone - the density at the part that we see is much ...


38

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 magnify your image a lot. And that makes the sun appear very very big. It only appears very big because it has been magnified. But if you are on Mercury, you ...


33

[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 easier to intuitively reason about.] Let's assume the top photo was taken when Earth-Mercury-Sun is a straight line (this will be very close to true in the top ...


28

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 as long as the star is burning. Once a star the mass of the sun is no longer able to sustain fusion, what is left is a white dwarf which is in fact much denser ...


21

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 conditions under which the comparison is being made. For a simple everyday example, at room temperature (and pressure) water is significantly denser than air. But ...


18

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 words, the Sun's angular diameter is much smaller as seen from Saturn. So that first picture doesn't really tell you much about the angular diameter of the ...


13

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), Mercury would look a lot bigger relative to the sun. So from the first photo, you can not derive anything as to how the sun would look from the surface of the ...


11

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 have significant enough amounts of it. This is even more pronounced with larger stars. VY Canis Majoris: "With an average density of 0.000005 to 0.000010 kg/...


9

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 there is a (roughly) 1 in 100000 chance that a randomly chosen transit of mercury will coincide with a transit of Venus. Since Transits of Mercury occur every 10 ...


8

The most-widely accepted hypothesis at the moment is that Mercury was struck by a large impactor that removed a significant fraction of its mantle (I believe this theory was originally proposed by Cameron & Benz in 1987, and the qualitative theory hasn't changed very much). For planets that are close to their parent stars (such as Mercury), the collision ...


8

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 a massive body. This calculation takes into account that the two objects have Keplerian orbits where the objects begins with at least the orbital speed of the ...


8

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 moon were too close to the planets, it would fall within the Roche limit and be torn apart by tidal forces. If a moon were too far from the planets, it would fall ...


8

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, Jupiter, and to a lesser extent, all the other planets, make Mercury's orbit precess by over 500 arcseconds per century. A key problem of the latter half of the 19th ...


7

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 & Stevenson (2006) have explored the possibility of a prior Venusian moon, starting from the assumption that Venus would not have been able to avoid a giant impact. ...


7

Both effects combined. Day being longer than year would just have retrograde motion of the Sun on sky, but no direction change. Variation of the distance alone happens on Earth, and we have no such effect. But the combination of both factors, in the precise amount they have on Mercury, makes this effect happen.


7

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 case of the Earth) that counts, not the outer reaches of the atmosphere. The surface is a clear-cut, non-arbitrary boundary. The atmosphere? They can extend a ...


7

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 about the causes of Mercury's composition: Early in the Solar System's history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass and ...


6

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 rotation. They used a new technique that involved bouncing a radio signal sent from a ground telescope in California off the planet and then catching it again in West ...


6

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 have far too many projects on the go. As you correctly say, it's blazingly obvious that Mercury is highest in the sky at lunchtime, and the code I wrote to ...


5

I've just rewritten this answer - @MikeG caught a glaring error by pointing out a really basic handy relationship called the Rayleigh criterion. \begin{align} {\theta}_R \approx1.22 \frac{\lambda}{D}. \end{align} It's better to read the (or any) article, but very briefly, the angular resolution is roughly the ratio of the wavelength to the diameter of a ...


5

Mercury's angular diameter on transit day will be 12 arcseconds. A camera obscura using a 12 mm aperture could resolve it; one lens from +0.75 diopter reading glasses, if you can get them, will project a bright 12 mm image of the Sun at a distance of 1.33 m. Note that a larger aperture or a shorter focal length will make the Sun image hotter than direct ...


5

Some things we know about Mercury's orbit: Semi-major axis: 0.387 AU, about 57.9 million km Eccentricity: 0.205 We can calculate the semi-minor axis, $b$ from the equation $$e=\sqrt{1-\frac{b^2}{a^2}}\to b=0.379\text{ AU}$$ We can also calculate the distance to the focus from the center of the ellipse, $f$, as $$f=ae=0.078\text{ AU}$$ I used Mathematica's ...


5

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 planet's radius is its core. Mars also has more sulfur in it's core and more Iron in it's mantle than Earth, suggesting that it probably didn't mix as well as ...


5

usrLTK's answer provides a lot of good details and in particular explains why Mercury wouldn't have much of an atmosphere. Let me complicate the picture a little by pointed out that some recent research indicates that magnetic fields may not be the guaranteed, automatic atmosphere-protection devices that conventional wisdom suggests. In particular, Gunell ...


5

Mercury's orbit is highly eccentric: 0.21 according to Wikipedia. Therefore, the actual time between repeating occurrences will vary depending on the year. If you were to perform your calculations for many periods, the average should approach the value given by Stellarium. The theoretical synodic period, using the sidereal period of Earth and Mercury, is ...


5

Thanks to the Mercury transit, you can measure the parallax from the Earth. That happens due to TRACE , which tracks the transit of Mercury along the polar diameter of the Earth. During that tracking, the transit of Mercury goes like that: [ Now notice that, if TRACE remained stationary, the transit would be a straight line. So, if you calculate the ...


5

The size of the object (i.e. Sun) on the Sky doesn't depend on how big is the body you are sitting on (i.e. Mercury). It only depends on a distance from you to this object and its size. So the size of the Sun when viewed from Mercury depends on the distance from Mercury to the Sun, and size of the Sun. Similarly when viewing Mercury and Sun from some ...


5

See these questions on photo.stackexchange.com. If you use a telephoto lens so that the sun fills your field of view, it will magnify the size of other objects depending on how far away they are from the lens. So while Mercury looks very close to the sun in that photo, in reality it is between 46 and 70 million kilometers away from it (closer to Earth). ...


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