Reading an Orbital Diagram

On Where is Ceres? and Where is Vesta? there is a javascript applet that shows the current orbital positions of each asteroid. This is a very rough depiction (ie, note that the scale is pretty extreme so what appears close is not all that close!) so please do not misinterpret what you "see" if you fast forward the applets. This page will go into detail on how to read the applets and other orbital diagrams that you might come across (like figuring out the phases and rise/set times of the moon!).

The above graphic is a simple drawing showing the Sun, Earth's orbit (blue circle) and Earth (teal ball), and an example orbit (that can represent the orbit of any other planet, asteroid, or comet in our solar system). Although there is an orbit, we can really generalize and say that any object off in that direction, like the distant stars, will also be visible. Here, we are looking at the solar system from above. The earth rotates counterclockwise, so you can see where the person is on earth at noon, sunset, midnight and sunrise.

At noon, with the sun dominating the sky, it doesn't matter where an object is on its orbit because we won't see it. But just after sunset, if an object is along the part of the orbit just to the left of the sun, we see the object in the evening also in the west. An object along the portion of the orbit in the lower left, is visible in the early evening in the east.

Now as the earth turns, an object in the early evening in the east will slowly move across the sky so that it is in the west at midnight. Now by midnight, more stars have rotated into view over in the east. Objects on the orbit in the lower right are also visible in the east around midnight.

But again, as the earth slowly turns, those objects slowly move across the sky and are in the west shortly before sunrise. And any objects on the orbit in the upper right or in that direction, rise just before the sun.

Before we relate this to the orbital diagrams on Where is Ceres? and Where is Vesta?, let's take a look at what the graphic is showing...

There is a main window that shows some colored ovals and dots. You'll notice today's date in the lower right corner, the target object name (in these cases, Ceres & Vesta) in the upper left, and the current distance from the asteroid to the Earth and the sun. There is a slider bar on the x-axis (bottom) and y-axis (right) of the main window. Below the window, there are options for changing the date, advancing the date by certain increments in steps or continuous mode, labels, zooming in/out, and saving a snapshot of the graphic. The colored ovals are the orbits of the inner planets and the dots are the current positions. If you look carefully, you'll see that the orbit for Earth is completely white but that the others are grey and white (while the asteroid's is blue and turquoise). This is because we use the Earth's orbit to define the plane or ecliptic of the Solar System. Since the other planet's (and comet's and asteroid's) orbits are slightly tilted (or inclined) to the Earth's, this means that part of their orbit is below (grey, blue or red) the ecliptic and part of the orbit is above (white, turquoise or pink) the ecliptic.

Let's talk about the slide bars on the right side and bottom. When you first open this page, the default orientation is from a perspective slightly above the Solar System. There are two lines that are half bright yellow and half dim yellow to help keep you oriented. The one up/down shows the above/below. Move the slide bar on the right side and you'll notice that your perspective above or below the Solar System changes, but that up/down line stays the same, except when you have the slide bar all the way to the bottom (looking at the Solar System from directly overhead) or to the top (looking at the Solar System directly from underneath) so the up/down line disappears. If you move the slide bar directly half way you are looking at the Solar System from the side (not the best view, except for seeing how the orbits are tilted to the ecliptic).

The other yellow line lies in the ecliptic and (the bright yellow portion) points towards the vernal equinox. (Change the date to 21 March of any year, then look at Earth's position on it's orbit -- it will be on the dim yellow part of the line. Notice that if you look from Earth to the Sun, the Sun is in front of the bright yellow portion. Now change the date to 21 September and the Earth is now on the bright yellow portion and the sun is in front of the dim yellow portion of the line.) Move the slider bar below the window and now we are looking at the Solar System from different points around it.

Now, that we know how the sliders work, we can adjust the orbital diagram and orient ourselves to answer our question on where to look for Vesta and Ceres in the sky. Be sure to change the date back to the current date. Move the right slider so you are looking at the Solar System from nearly overhead and move the bottom slide bar so that Earth (now this is going to sound strange) is straight below the Sun. You might want to zoom out to include the orbits of Jupiter. During late-May 2007, Vesta is opposite in the sky from the sun (sun-Earth-Vesta are lined up). This is opposition and Vesta is up all night long. As we continue into June, Vesta is more on the evening side (be sure to readjust the slider so that earth stays "below" the sun) Ceres, on the other hand is to the right of the sun and during the summer of 2007 is a morning object. But as we catch up to it, Ceres will be at opposition in early November.

Archive: Deep Impact -- Tempel 1

Now, that we know how the sliders work, we can adjust the orbital diagram and orient ourselves to answer our question on where to look for Tempel 1. Be sure to change the date back to the current date. Move the right slider so you are looking at the Solar System from nearly overhead and move the bottom slide bar so that Earth (now this is going to sound strange) is straight below the Sun. You might want to zoom out to include the orbits of Jupiter and Saturn. During late July-early August 2004, Tempel 1 was behind the sun making it impossible to see. If you move the dates forward into October, you'll see the Earth continue around the sun and as we look towards Tempel 1, we'll see it on the right side of the Sun -- Tempel 1 is now a morning object. It rises shortly before the sun, but each night, it rises a bit earlier. As we adjust the dates forward into the spring of 2005, we see that the comet seems to be getting behind the earth (be sure to readjust the slider so that earth stays "below" the sun) so that the sun-earth-comet line up in April, that's called opposition and the comet is visible all night. As we get closer to impact, the position of the comet relative to the earth and sun continues to shift (again, be sure to readjust the slider to keep the earth "below" the sun) so that the comet is to the left of the sun which means that it is visible in the evening sky shortly after sunset.

The four images at right show screen shots of the orbital diagram on different dates. All are setup so that the view is from overhead and with earth at bottom. Notice how the position of Tempel 1 changes in relation to the Earth and Sun. In the top image (late July 2004), T1 is in line with the sun (conjunction) so it is essentially not visible. By October (2nd image, mid-October 2004), T1 has moved in its orbit and is now to the right of the sun. It is a morning object rising shortly before the sun. By early April (2005), it is opposite the sun in our sky (opposition) and is visible all night. By Early July (2005), T1 is just to the left of the sun, so it sets a few hours after sunset.