Saturday, June 2, 2012

The Solar System at Easter Island

By now, you've probably heard about the transit of Venus on June 5-6, 2012. I wrote a short post (here) describing it and a second one (here) describing one method to estimate the distance to the Sun using the transit. Although Chile won't see the transit, a group of us will be traveling to Easter Island to watch it from there (technically *have* travelled since this post was scheduled in advance).

We've created a whole outreach plan for our time at Easter Island, which includes a two-day workshop at the local museum (with talks by the astronomers), school visits the following day, observing the partial lunar eclipse, and the transit of Venus itself on June 5.
One of the things I prepared was an activity on the scale of the solar system specifically for the residents of Easter Island.

First off, here's a map of Easter Island:
Edited from the topographic map of Easter Island created by Eric Gaba available on Wikimedia Commons.

Note that I've placed a partially transparent band through the island and also on the zoomed inset. This was printed on a large poster-type paper and brought to the island. What we do here is place the Sun at the single town on the island, Hanga Roa, and place Pluto at the farthest end-- Cape Cumming. That sets our end points and now the fun can begin.

I printed out a set of planet images, which you can see below. The activity is fairly straightforward: we talk about the planets to the group of students/kids/adults and ask them to place them where they think they lie on Easter Island given the two end points we've defined. It's like a game of 'pin the tail on the donkey', but with some science involved. How close to the actual locations can they get? Will they sort by planet size, coloration, or surface features? How does this compare to extrasolar planetary systems we've discovered? I'll have more to say on how this goes on a future post...
Planet images from a variety of NASA facilities including HST, Pioneer, Voyager, Terra, and Messenger. From top to bottom and then left to right: the Sun, Jupiter, Saturn, Earth, Neptune, Uranus, Venus, Mercury, Mars, and Pluto.

So, what's the answer? It helps if you know the distances to the planets, of course, but the Titius-Bode law can also be applied here without having to deal with the actual numbers. That law is an empirical observation that the orbit distances of the planets are roughly a factor of two from each other. For example, Uranus is 20 astronomical units (AU) from the Sun, Saturn is 10, Jupiter is 5. This doesn't work perfectly: there is no planet at 2.5 AU (that's the asteroid belt) and Neptune is at 30 AU so it gets missed, but it's a nice rule-of-thumb. Here are the approximate planet distances from the Sun:
  • Mercury: 0.4 AU
  • Venus: 0.7 AU
  • Earth: 1.0 AU
  • Mars: 1.5 AU
  • Jupiter: 5.2 AU
  • Saturn: 9.6 AU
  • Uranus: 19 AU
  • Neptune: 30 AU
  • Pluto: 40 AU (I know it's not a planet; you can replace this with "Kuiper Belt Objects")

So, how should our answer look like? Here it is (remember that you can click to see figures larger):

In this scale, the terrestrial planets are all within a kilometer of the museum (where we placed the Sun) and the giant planets are spread throughout the island. Interestingly, there are two small peaks in the island that correspond roughly to the location of Jupiter and Saturn (Tuutapu and Maunga O Tu'u).

The planet disks are not to scale in this image as we need the attendees to be able to hold them and place them on our poster printout. We can easily calculate the sizes, though. The separation between the Sun and Pluto is 40 AU, but at this scale it is about 20 km. In other words, each kilometer is about 2 AU. Inverting that with the knowledge of how much an actual AU is (150 million kilometers) allows us to convert any astronomical distances we have to Easter Island-scale.

So how do the planets size up? The Sun is big: about 4.7 meters across in this scale. Jupiter, the largest planet, is a factor of 10 smaller, or about 47 cm. The Earth is slightly over 100 times smaller than the Sun and so is a tiny 4 cm in diameter. Imagine that Easter Island was completely empty except for one 4.7 meter shining ball and a few other balls just a few centimeters across spaced kilometers apart. Space is empty and objects it in are very far apart from each other.

A last curious object is the nearest star to the Sun: Proxima Centauri. This star, part of a triple system, is about 4.2 light-years away or 1.3 parsecs (remember my discussion on astronomical distance units on this post?). That corresponds to over 265,000 AU-- quite far! In this scale, Proxima Centauri would be about 135,000 km away from Easter Island. That number is hard to grasp as it's larger than the Earth. You would have to go around the Earth nearly 3.5 times to reach that distance. On Easter Island-scale, this distance is actually about one third (1/3) of the separation between the Earth and the Moon!

Space is truly big. I hope that with this activity we can impress that upon the children living in Easter Island. Other science goals of this particular activity include learning the relative positions of planets in our solar system and comparing ours to extrasolar planet systems.

Our team --- Team Hetu'u (for the rapanui word for 'star') --- consists of members from Universidad de Chile, Universidad Andres Bello, Universidad Catolica, and Cerro Tololo. This is one of several activities our team has prepared. I'll have more to say about how everything went in a week or so. For now, here's a post on the American Museum of Natural History webpage about our work.

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