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

I’ve seen a lot of photos showing Mercury in front of the Sun when it passes by and you can see just how tiny it is in comparison.

Here’s a great example:

So I’d expect if I was travelling towards Mercury in that photo that as the planet got bigger in my view so would the Sun and eventually I wouldn’t be able to see the edges of the sun because it’s so big at this distance... makes sense that an object gets bigger the closer you get to it!

However what confuses me is that if you were on the surface of Mercury... this is the view you’d get where it’s about a 6 times larger than when viewed from Earth.

So how is this so small when it’s taking the whole view up when we’re viewing it further away from mercury in the first photo? But we’re closer in the second photo but now the sun is smaller... How is the first photo possible (it’s an actual photo taken by NASA) why isn’t it taking up the whole sky in the second photo when you’re actually on the planet?

• The Earth would also look tiny with an image like the first one. – J.G. May 8 at 15:02

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 don't need to magnify the image of the sun. The sun on Mercury is about 5000 arcseconds across. Big, but not filling the sky. That is just because it hasn't been magnified

• So the simple answer would be "because the first photo is edited"? – user17915 May 7 at 0:53
• Or in other words, had the first photo not been edited, the sun would have looked much smaller (maybe slightly bigger than what it looks like from the Earth and slightly smaller than what it looks like in the second photo) and Mercury would not have been visible at all? – user17915 May 7 at 0:54
• @user17915 not really 'edited', but 'shot with much smaller FOV' – John Dvorak May 7 at 4:58
• @user17915 using the zoom on a camera is not editing. Otherwise I would technically be editing reality every time I look through a telescope :o – Fred Stark May 7 at 6:44
• -1 because you don't mention field of view. This is the important photographic principle that is at the heart of the effect in question. This is also how you get those wolf-howling-at-the-moon photos where you have a wolf or something in the foreground and a giant Moon behind it. These are taken by positioning the camera some distance away and then zooming in on the wolf to get it back to "normal" size. The small FoV magnifies the distant background enormously. – Oscar Bravo May 8 at 11:40

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 first image makes it appear.

You also need to bear in mind that objects in space tend to lack "depth cues": in the atmosphere, objects further away will become blurrier and more "washed out" due to scattering. In space, this does not apply.

• "Density of crowds" was exactly the case I thought of when I saw the picture in the question. – chrylis -cautiouslyoptimistic- May 6 at 12:18
• @chrylis-onstrike- sadly, "Density of crowds" has taken on a rather different meaning recently in the USA – Carl Witthoft May 6 at 15:00
• This effect is also used in cinematography in the dolly zoom. See for example this animated gif of a famous jaws scene. – marcelm May 8 at 8:33
• With a good telescope around Neptune, you should be able to make a picture with the Earth soaring in front of the Sun. – gerrit May 8 at 9:00

[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 photo). Let's set up the same scenario on Earth, but scaled down.

The diameter of Mercury is 4900km, the Sun is 1.4 million km. This is a ratio of 285:1. Mercury is 40 million km from the Sun, Earth is 150 million km (ignoring eccentricity, etc). Mercury is about 73% of the way to the Sun from here.

A basket ball is about 240mm in diameter. Something 285 times smaller than that is just under one millimetre. That's about the size of a spider mite or a large grain of sand.

In order for the ball to occupy 0.5 degrees of your vision, it needs to be 25 metres away. That's the length of a basketball court, which are probably not in use right now, so lets go down to one. Place the ball under one hoop, then put the spider mite on the tip of the "three point line" at that end of the court. It's magical, so it floats 120mm from the ground. Handily, one hoop's three point line is almost exactly 73% of the way to that hoop from the one at the far end. Now go and lie down under the other hoop and position your head so the mite is in front of the ball from your perspective.

When you look at the tiny mite out there on the far side of the centre line, and the ball at the far end, you have pretty much the same scenario as Earth:Mercury:Sun during a conjunction. Notably, you probably need a telescope to see the mite at all under any kind of normal lighting. Looking at the mite "transiting" the ball though your telescope, doesn't the magnified ball look enormous with the mite floating in front of it like a speck?

Now, go back to the mite and look at the ball from its perspective (don't stand on the poor thing). The ball doesn't look hugely bigger from here, does it? You can probably read more of the writing on it and see the texture, but it's still not taking up all of your vision.

This is because you're not using your telescope any more. If you use the same telescope you used to look at the mite and ball, but stand almost 4 times closer than you were originally, the ball will look even huger.

If you wanted the Sun to look the same size as a basketball does when holding it with the surface 0.5m from your head (2 diameters), that's about 3.5 million km orbital diameter: much, much closer than Mercury really is.

It's twice as close as the Parker Solar Probe will get: the PSP will orbit at 8.5 solar radii, so it'll see the Sun as about size of a basketball that you can't quite touch (just over a metre away).

2 diameter's separation is, however, roughly how Io:Jupiter is set up (Io orbits at about 350000km from the Jovian "surface", Jupiter is 140000km in diameter), so from Io, Jupiter really is huge in the sky, like a basketball in your hands would be.

Metis, at only 58000km from the clouds (orbiting at about 128000km from Jupiter's centre), would see Jupiter like you would if you put your eye about 100mm from a basketball: Jupiter really would fill the sky.

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 Sun as seen from Mercury. If anything, it just shows how small Mercury is compared to the Sun.

(The image is the 2669 Saturn transit as seen from Uranus. I hope we'll have developed space travel by then that we can go see it ;) )

• Is there no actual picture of an Earth transit from some probe available? – Hagen von Eitzen May 6 at 18:10
• @HagenvonEitzen, no, nothing's been in the right place at the right time. Cassini observed a Venus transit from Saturn, Curiosity observed a Mercury transit from Mars, and STEREO B observed a Moon transit from deep space, and that's about it for off-Earth transit observations. As far as I know, the next opportunity for an Earth transit will be from Mars in 2084. – Mark May 6 at 20:10
• @Mark There should be plenty of Earth transits from the Moon before 2084, and at any arbitrary time if we are free to choose our vantage point :) – gerrit May 8 at 9:02
• @gerrit: Those'd be eclipses, not transits. (And yes, they did try to observe one once, with the Lunar Rover camera from Apollo 17, but the camera failed shortly after the astronauts left, so they didn't get any pictures.) – Sean May 8 at 22:59

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 planet.

• This answer really nails a key point. The OP assumes a misconception, that the size of the ball you're standing on will influence how big an object in the sky will appear. Here's another thought experiment: Stand on the Earth looking at the Moon. Now step onto an exercise ball. Now have the Earth disappear so you're only standing on the ball. The Moon will take up the same amount of the sky in all three cases. – JonathanZ supports MonicaC May 7 at 2:17

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 distance (i.e. from Earth), it purely depends how far you are from the Sun and Mercury, and how big they are (constant).

In fact if you are just few meters above the ground of Mercury, it is bigger than the Sun in your field of view. Way bigger.

If you are very far away (like orbit of the Earth), the apparent sizes ratio between two observed bodies is the same as ratio of their diameters. But that is not the case when you are closer.

Attaching a sketch of calculations, that should help you see it better.

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). That's between 33 and 50 times the diameter of the sun.

Imagine a hot air balloon in the air that is far enough away that it is 1/4 the size of the full moon. If you used a telephoto lens so that the moon filled the field of view the hot-air balloon would appear small in comparison. But if you were in the hot air balloon the moon would appear the same size as it would to the photographer without the telephoto lens and it wouldn't fill the sky.

Here's a cool article showing shooting a model on a hill with the moon in the background using a telephoto lens. The moon doesn't appear as big to the woman as it seems in the photos because her apparent size is being increased by the same amount.

https://petapixel.com/2017/10/26/shooting-portraits-giant-moon-using-1120mm-lens/

If you hold a marble at arm's length, it's about 1cm wide at 100cm away. The moon and sun are also about 100 times further away than they are big, so they are as big as the marble.

If you bring the marble at 20cm away from your eye, that's like being on planet mercury the marble is 6 times bigger.

If you shoot the marble at the big sun and take a photo of it 100 meters away, the sun would be 100 times bigger than the marble.

When you look at the photo of big sun and the small marble you would think "If i was on the surface of that marble, the sun would take up most of the sky"... It's just an illusion of perspective from the photo.