Harry’s “Comparative Astronomy” Pages

(Or: how much one can get out of drawing circles)


Our Solar System


Would you like to get an idea of what the distances in our solar system — and beyond — look like?

If you are like me, you would probably enjoy watching the distances and sizes of planets, the Sun, and their satellites, all drawn to scale in the drawings below (as close as possible, given the minimum computer graphic unit of one pixel).

For example, look at the figure below.

The Sun and the six largest planets are all drawn to scale (approximately). Earth and Venus (which are almost equal in size) are shown by merely one pixel. Uranus and Neptune, next, have a diameter of four pixels each (four Earth-diameters). Saturn is at nine pixels, Jupiter at eleven, and the diameter of the Sun is 109 pixels wide. Objects not shown in the figure, such as Mars, Mercury, Pluto, the Asteroids, and the newly discovered tenth planet, would require less than one pixel, so we cannot depict them.

Okay, you’ve probably seen a figure like the one above in several school texts or introductory astronomy books. The next one though is probably new to you.

In this figure the Sun's diameter (5 pixels) and Mercury’s, Venus’s, and Earth’s distances are drawn to scale. (What cannot be drawn to scale are the diameters of the three planets: we should devote much less than a pixel for them.) So this is what the distances of the three inner planets would look like compared to the Sun’s disk, if you could take a bird’s-eye view of this part of our solar system.

Not impressed? Let's go on.

How about putting the Moon’s orbit around the Earth in perspective in the above figure? Well, we can’t! The reason is that since we devoted five pixels for the Sun’s diameter, we would need about 2.76 pixels for the disk representing Moon's orbit around the Earth. So let's put the Earth at the Sun’s center, and see what the Moon’s orbit would look like, then.

Here, the Sun’s disk, the Moon’s orbit, and the one pixel representing the Earth at the center are all drawn to scale. Imagine, the several- days-long flight of the various Apollo missions in the 60’s and 70’s would not even get them to two-thirds of the Sun’s radius, assuming that they started at the center. (Well, they didn’t travel along a straight line, so maybe they could make it up to the Sun’s edge.)

What we cannot depict in the above figure is the Moon itself. Being smaller than the Earth, it would take, once again, less than one pixel. So let’s put in one figure the Earth’s disk, the Moon’s disk, and their mutual distance, all drawn to scale.

As promised: we have the Earth’s disk with a diameter of 15 pixels, the Moon’s disk with four pixels, while their distance comes to around 452 pixels.

If you are interested in comparing the distances of almost all satellites to their planets (always pictorially), look at this diagram.

Now, how about moving a bit further? No, I am not going to repeat the common drawing which shows all the orbits of all nine planets around the Sun. What I want to do is let us get a feeling for the “size of our solar system”. How could we define that? We may say that our solar system is a disk with radius equal to the mean radius of Pluto’s orbit. That happens to be a rather sloppy definition because Pluto’s orbit is extremely eccentric (it even crosses Neptune’s orbit at some point). Still, we need to think of our solar system as a circle, not an ellipse, so we’ll stick with Pluto’s average distance from Sun, which is about 5,900,000,000 km. (To see how much it varies, Pluto’s closest point to the Sun — its perihelion — is at 4,425,000,000 km, while its farthest point — its aphelion — is at 7,375,000,000. Maybe Neptune would serve better as defining the circumference of our solar system: its mean - min - max distances are as follows: 4,497,000,000 km – 4,456,000,000 km – 4,537,000,000 km.)

What known quantity should we compare Pluto’s mean distance against? How about Earth’s mean distance from the Sun? (You’ve already seen this distance above.) So here is what the Earth’s orbit looks like when compared to Pluto’s average orbit.

This time, the single pixel at the center is too large to represent the Sun’s disk. The latter would be a spot only 6.5% of a pixel wide in the above figure.

Isn’t it time now to move beyond our solar system?


Just how big does the Sun look when viewed from Mercury? From the other planets?

And, is there anything even more impressive than that?

Care to see how big is Jupiter’s Red Spot, compared to Earth?

Tired of watching my circles? Try these external links, then:


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