Of Particular Significance

Estimating the Distance to Jupiter in My Backyard

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 02/09/2024

Last night, using the methods I described as part of my check-it-yourself astronomy series, I estimated the distance to the planet Jupiter using nothing more than my eyes, a protractor, and a simple calculator. It took about 30 seconds of measuring something before and after sunset, and about 15 more seconds using my cell phone’s calculator. You can do it too, if you have clear skies over the weekend.

There are only two parts of the process:

  1. know which week to ask the question, and
  2. during that week, measure the angle A in the sky between the Sun and Jupiter.

Then the distance from Jupiter to the Sun RJS is the distance from Earth to the Sun RES times the tangent of the angle A

  • R_{JS} = R_{ES} \tan A

The reason is simple geometry, corresponding (for a general planet P) to the figure below.

Figure 1: When an outer planet is about 1/4 or 3/4 through its cycle between one opposition and the next, the Earth, Sun and the planet form a right triangle, which allows a quick estimate of its distance RPS from the Sun relative to the Earth-Sun distance RES.

When to Measure?

The day on which you should make the measurement is the one when Earth, Jupiter and the Sun make a right triangle as in Figure 1. You can estimate this date quite easily. Once every 13 months or so, Jupiter is at “opposition”, meaning that there is a straight line connecting the Sun, Earth and Jupiter, as at the far left of Figure 1. At opposition, Jupiter is directly overhead at midnight. Let’s call the time between oppositions the length of Jupiter’s cycle. Then, about 1/4 or 3/4 of the way through the cycle, the three heavenly bodies form a right-angle triangle, drawn in Figure 1, and the angle between Jupiter and the Sun as seen from Earth is enough to determine the ratios of the triangle’s sides.

Looking at Figure 1, you can see that if RJS were just a little larger than RES, the angle A would be close to 45 degrees. If RJS were enormously larger than RES, then the angle would be very close to 90 degrees. But wandering into your backyard just after sunset, and looking up, reveals the angle to be much larger than 45 degrees, yet clearly less than 90.

Figure 2: Last night, 30 minutes after sunset, Jupiter was close to overhead, about 75 or so degrees from the point where the Sun had set, to the right of the image.

How to Measure?

This is a bit harder, but how hard it is depends on how accurate an answer you are seeking.

The first problem is that you can’t see Jupiter at the same time you see the Sun — the former is too dim and the latter too bright — so you can’t measure the angle between them directly. You need to watch where the Sun sets. Call that the “sunset point”. Then you need to measure the angle between Jupiter and that point of sunset, let’s say, 30 minutes after sunset. Next, you need on an additional angle that accounts for how far the Sun has moved below the horizon during that 30 minutes. (Perhaps the best way to do this is to look at the Sun 30 minutes before sunset, estimate its angle away from the sunset point, and then, assuming the Sun moves by that same angle in the next 30 minutes, add that angle on to your 30-minutes-post-sunset measurement of Jupiter.)

You’re not going to get a perfectly precise answer, and that leads to the second problem. RJS is quite sensitive to the angle A — the further out the planet is from the Sun, the worse this problem becomes. So if you don’t measure the angle accurately, your estimate of RJS may be rather far off. (Note: Even if you do measure the angle accurately, this method cannot get you a precise answer, because the whole method is based on idealizing Earth and Jupiter as traveling at constant speeds on circular orbits, which is it an approximation of the truth.) This can be seen in the graph below, which shows that as A approaches 90 degrees, the inferred planetary-sun distance grows very rapidly.

Figure 3: The relation between a planet’s distance from the Sun, relative to Earth’s, and the angle that it forms relative to the Sun at the 1/4 or 3/4 stage of its cycle between oppositions. The further is the planet from the Sun, the more precisely the angle must be measured to obtain a good estimate of its distance.

My Estimates

I started with a rough qualitative estimate, with no intention to be precise. Just a quick glance at the sky revealed that the angle between Jupiter and the point of sunset was less than 80 degrees and more than 60. Accounting for the Sun’s motion after sunset, that put the angle A between 85 and 65 degrees. That only tells us, as shown in the dashed lines of Figure 4, that the Jupiter-Sun distance lies between 2.2 and 11.6 times the Earth-Sun distance. Still, it already reveals that the ratio of the distances is neither 1.1 nor 100; Jupiter orbit is significantly, though not spectacularly, larger than Earth’s.

Then I measured things a bit more carefully, taking about thirty seconds to do it, and found the estimate shown in the solid lines of Figure 4: the angle is somewhere between 77 and 82 degrees. That’s accurate enough to tell us that RJS lies between 4.3 and 7.1 RES . Although that’s still not precise, it is much better than my first estimate.

Figure 4: My first estimate, given by glancing at the sky, lies between the dashed lines, and determines Jupiter’s position only roughly. My more careful estimate of the angle A lies between the solid lines. The true Jupiter-Sun distance (which varies as Jupiter orbits) is given by the red bar.

In fact Jupiter’s orbit isn’t circular, nor is Earth’s — RJS moves between 4.95 and 5.46 times the average of RES , as shown by the red bar in Figure 4 — so it’s not possible to achieve better than 10%-20% precision using this method. But it’s pretty good for something that requires nothing more than pre-college math and the naked eye! This approach works that much better for Mars (because the angle for Mars is smaller, and the result less sensitive to how well you measure the angle). Conversely, it works less well for Saturn, Uranus or Neptune.

If you like this kind of challenge, try it, and let me know if you succeeded in getting a more precise measurement of the angle and what you found for RJS. Or if you’re a science teacher, give it as a challenge to your students.

A shortcut — less fun, but much more precise — is to look up the positions of Jupiter and the Sun in the sky, using the coordinates on the sky known as “right ascension” and “declination”. This information is easily found on many websites, such as this one and this one. I won’t go into the details here, but it isn’t hard then to extract the angle between Jupiter and the Sun from this information, and obtain a more precise estimate of RJS.

Of course, so far we’ve only found the ratio of two distances, RJS /RES. If we want to know RJS in miles or kilometers, then we need first to measure RES in miles or kilometers. Fortunately, this isn’t that hard to do… using meteor showers. But that’s another story.

Final Thoughts

The distance REJ from Earth to Jupiter changes dramatically as the two planets orbit the Sun, from a maximum of RJS +RES when the planets are on opposite sides of the Sun to RJSRES when Jupiter is at opposition. But last night, using the right-angle triangle of Figure 1, we know that the distance was given by the secant (=1/cosine) of the angle A:

  • R_{JS} = R_{ES} \sec A

For angles close to 90 degrees, secant and tangent are almost equal, and so it turns out that my estimate reveals that Jupiter is currently between 4.4 and 7.2 times further away than is the Sun.

With a telescope, there are other methods for obtaining RJS, which you can infer from Figure 1. Jupiter appears larger near opposition, when it is as close as it gets to Earth, and smaller when it lies almost at the other side of the Sun; comparing its angular size at these times would tell you RJS relative to RES . The fraction of the planet which is sunlit from our perspective at various times in the orbit also reveals RJS .

I hope that some of you go out over the weekend and give this a try! It just takes a few minutes. The same approach works for Mars and Saturn, the other naked-eye outer planets, and a similar approach works for Mercury and Venus. But in each case, we have to wait for the right week.

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5 Responses

  1. If you live on a coast, on a very clear night, you can tell the time the sun sets to within a second. You watch for the last little bead of direct sunlight to disappear. It’s very distinctive. You start a stopwatch and wait for Jupiter to disappear. You’ll have to correct for the radius of the sun which adds about a minute. I haven’t tried it but I’ll bet you can get a pretty accurate Sun/Jupiter angle reading by using the sunset time difference.
    As an aside, there is a stairway from the beach up to the top of a ~75 foot bluff in Redondo Beach CA, near where I live. From the beach, I watched the last vestige of the sun disappear and then ran up the stairway and looked again. I saw two sunsets that evening.

    1. It’s a very good idea, but it doesn’t quite work so simply as you might think. That’s because the angles at which things travel across the sky aren’t independent of both terrestrial and celestial latitude. You can check this for yourself; some websites list setting times for both the Sun and Jupiter, and you can check that they aren’t identical — and that they therefore don’t give the Sun-Jupiter angle — if you compare Anchorage to San Francisco to Acapulco to Lima.

      Now maybe there’s a smart way to correct for this that I have not noticed, if you’re clever about geometry of a spinning Earth that is tilted relative to the ecliptic…?

  2. What a delightful article! Thank you, Matt Strassler. While I cannot say that I’ll actually try this, you now have me thinking about how to do such a measurement every time I see a planet in the sky!

  3. Hi Matt,
    a quick note, in the first image you talk about RPS (I guess for Planet-Sun), but in the article you switch to RJS (Jupiter-Sun).


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