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“How far can you see with it?” is a typical question asked by those who are unfamiliar with telescopes. People who ask this question are not aware of the enormous distances one can see with just the naked eye. We’ll discuss just how far at the end of this article. Astronomers, both amateur and professional, routinely deal with enormous distances. This article won’t explain how to make precise calculations of the distances to the planets, stars and beyond, but will give you a better understanding and appreciation of astronomical distances.
Let’s start with the Earth. The distance from the center of the Earth to its surface is 3820 miles or 6371 kilometers and its circumference at the equator is about 24,000 miles or 40,000 kilomters. It’s no coincidence that the Earth circumference is a round number in kilometers because the kilometer was originally defined as 1/10,000 of the distance from the pole to the equator. The next distance is the 240,000 miles (400,000 kilometers) from the Earth to its Moon and then the average distance from the Sun to the Earth: 94 million miles (150 million kilometers).
It’s difficult enough dealing with hundreds of thousands of miles; how can you handle over a hundred million? The answer is: you don’t – at least not directly. The average distance between the Earth and the Sun (it has to be an average because the Earth’s orbit is not a circle and constantly varies) has been defined as one “astronomical unit” or A.U. Distances such as those between the various planets and the Sun are usually expressed in astronomical units. For instance, Mars whose orbit’s radius is a little over one and a half times that of the Earth has an average distance from the Sun of 1.52 A.U. Venus whose orbit is within that of the Earth averages only 0.723 A.U. from the Sun.
The following is a list of the planets and their average distance from the Sun in Astronomical Units:
| Mercury | 0.387 |
| Venus | 0.723 |
| Earth | 1 (of course) |
| Mars | 1.52 |
| Jupiter | 5.20 |
| Saturn | 9.54 |
| Uranus | 19.2 |
| Neptune | 30.1 |
| Pluto | 39.4 |
By expressing these vast distances in Astronomical Units, even far away Pluto – billions of miles away – can be easily compared to the other planets. But tables like the one above create an illusion of a crowded Solar System especially when they display the planets with both correct orbits and relative sizes but not using the same scale. To get a more accurate picture, let us imagine shrinking the Earth down to the size of a pea. At the same time, let the rest of the Solar System be proportionally reduced in size.
The Sun will be the size of a beach ball. The first planet Mercury be 200 feet from the Sun and will be the size of a pinhead. Then, 300 feet away from the Sun will be Venus, represented by a pea only slight smaller than that representing the Earth. In this scheme of things, the Earth is 500 feet from the Sun. About 20 inches from our pea revolves the Moon. Mars is the size of a grape pit and is 700 feet away from the Sun. Skipping over the asteroids, at a distance of over 2,000 feet from the Sun we come to Jupiter, the largest planet. A tennis ball represents Jupiter, while Saturn, which is over 4,000 feet away from the center, is only slightly smaller than a baseball. The next planet Uranus is over one and a half miles from the Sun and is the size of a ping pong ball. Neptune also is a ping pong ball, but one that is over two and a half miles from the Sun. Finally, Pluto is over three miles away from the Sun and the size of a sesame seed.
The general idea is that we have within a radius of over three miles a space filled by only an occasional golf ball and a few widely separated peas. Most of the Solar System is empty space. By the way, on this scale, the nearest star is 25,000 miles away.
As you can see although an Astronomical Unit is very useful for dealing with distances within our Solar System, it’s not going to help us much when we go out to the stars – even the nearest ones. For these distances we need a much larger unit of distance: a light year. A light year is the distance that light travels within a single year: 5 trillion 869 billion miles and change (about 9 trillion kilometers). The nearest star – alpha Centauri – which can only be seen from the Southern hemisphere is 4.3 light years away. Most of the brighter stars than can be seen from the Earth are no more than a few hundred light years away. A light year is not only a way of conveniently expressing large numbers but is related to a fundamental fact about the stars. Let’s take a look at the star Rigel from the constellation of Orion which is over 500 light years from us. The light that is entering our eyes while observing it left at least 500 years ago. When we gaze into the sky we are not only look across great distances, but also look into the past. By using light years, we are reminded that the farther the star is away from us, the longer we are looking into the past.
The question was posed at the beginning of this article: “how far can one see?” If you take a look at the constellation of Andromeda during a particularly dark night, you might see a small patch of light that is designated M31 (see skytour - andromeda). This is the Andromeda galaxy which is similar to our own and is the most distant object (2.9 million light years away) that can be seen with the naked eye.
Many American are unfortunately not yet comfortable with kilometers. A kilometer is about six tenths of a mile. A better way to picture it is as ten American football fields lined up one after the other.