We are used to saying that the Sun/Moon/planets/stars "move across the sky". That's because we interpret everything relative to us. We live on a rotating system, so really the stars are not moving - we are!
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Let's orient ourselves - pretend you are looking down on the Solar System from above so that the Earth goes counter-clockwise around the Sun. From that viewpoint, the Earth would also rotate counter-clockwise on its own axis. So, since the East rotates toward the Sun first, we see the Sun "rise" in the East. Later, the West will rotate around so the Sun "sets" in the West. |
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The stars also "rise" and "set" just like the Sun and the moon. How can we detect the movement of the stars? Use a simple 35 mm camera and a sturdy tripod. Just point the camera at the sky, use the BULB setting, and let it sit there for as long as you want (at least 10 minutes to get some trailing of the stars). If you are not in a very "dark" location, the "sky glow" will creep onto the film, and you won't be able to leave the camera open as long - if you are in a dark site - go for an hour or more!
What will you see in the images? Well, it depends on where you point the camera. The stars appear to move because we in fact are rotating around the Earth's axis. Luckily, there is a star up in the sky very near the point where the Earth's North Pole points .. the North Star (Polaris). Thus, as the sky rotates above us (meaning, we turn against the background of the stars), the North Star should not move very much. (It moves just slightly because the axis is not quite pointing at the star - but basically the North Star is always in the same point in the sky for us). What about the stars near the North Star .. they will rotate around the North Star. If you face the North Star, the stars will appear to rotate counter-clockwise around that point.
As you get farther from the North Star, the stars still rotate, but their trails appear to straighten out (and the tracks seem longer). These two effects are connected - the star farther away in the sky makes a larger circle with respect to the North Star .. so a segment of its path should appear straighter than near the North Star. Also, since the distant star has to go all the way around its bigger circle in 24 hours - it has to cover more "distance" along that path in a 10 minute period than a star that is making a small circle around the North Star.
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| [Here is a composite of a sky strip from the North horizon to the South Horizon.] |
The strip above shows 7 images that were blended together to form a "sky strip". On the left is the North Horizon - then the camera is tilted up slightly, and another shot is taken .. then tilted again .. another shot .. etc. Finally, the camera is pointing at the Southern Horizon.
There is more detail in the strips below, but let me give you some pointers to watch for :
[If we could see where the Earth's South pole points in the sky .. the stars would curve strongly around that, same as with the North Pole. There is no "star" where the South pole points, but there is a "clear space" in the sky in the general vicinity (not many stars, by coincidence).]
Here are two images (one in color and one in greyscale) presented vertically so you can scroll up and down a little better. The greenish tint is a result of the low light and the sky "glow" (lights from the nearby towns), and is an artifact of the film (the sky was not green!). The next time I get to a really dark site, I will update this with better photographs (and hopefully a longer exposure time).
Color sky stripComposite Color images - 10 minute exposures using Fujicolor800 film - 50mm lens f/stop=1.8  |
Black and White sky stripSame images were scanned, but only greyscale .. then inverted (black to white) to bring out the faint detail.  |
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Closer look at the Southern Horizon? With a sideways picture, and a slightly darker sky, we could more clearly see the "downward" curve of the southern stars. (I'm still waiting to get a nice dark sky and a long time exposure!) [Click on the image to see the larger picture.] |
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If you have a 35 mm camera and some color film - I highly recommend you taking some "star trails" pictures. You want to find a dark sky and it is better to shoot up toward the sky overhead (since there would be less of the horizon glow that way). I'm still playing around with what speed films work the best : slow films have good resolution, but might not pick up some of the fainter stars, higher speed films pick up the fainter light, but don't have as fine a resolution. I've used 400 and 800 speed film with good results - I'm going to try some 100 speed and see what I get.
Why color film and not black and white film?When we look up in the night sky, we basically just see "white" stars. Sometimes we get the feeling that a star is a different color than white (more reddish or blueish). The problem is that our eyes don't perceive color very well in low light situations (dark night sky!). But, film can! The light accumulates on the film, and the colors can be resolved. I tried to search my film archives .. but I obviously have to do some better organizing .. here is a part of an image that will show slightly different colors. (I'll try to get a better example here!) |
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Most of the orbits of the planets around the sun fall along the same plane (large flat surface). A few are tilted a little bit from the plane, and Pluto is tilted alot! The Earth's Moon also moves basically along that same plane. Thus, when the Sun moves across the sky, or the Moon, or the planets - they all bascially follow the same path in the sky (with slight variations). We call this path the ecliptic.
If you imagine the Earth's equator extended outward into space .. there will be a "line" across the sky - we call that the equator.
So, we have one giant circle in the sky which is the ecliptic (where the sun/moon/planets) travel, and another giant circle representing the Earth's equator projected up into the sky. Do they cross? Yes, at two times of the year : the Vernal (Spring) Equinox, near March 21st, and the Autumnal (Fall) Equinox, near September 21st (you'll see than in an illustration later on the page).
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Which one is responsible for the star trail curves? The stars appear to move because of the Earth's rotation, right .. thus, it must be the equator that defines how the stars curve - "above" the equator the stars apear to curve around the North Pole (or the North Star) - "below" the equator, the stars appear to curve around the South Pole (or that big "gap" in the stars). |
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Can we see an example of the planets being in different orbits? Well, this is tricky. The ecliptic is tilted relative to the equator (since the Earth's axis is tilted relative to the plane of the Solar System). So, two planets might have orbits that would put them in a path very near each other lined up with the ecliptic, but because of the rotation, they will appear to be separated in the sky if we let the planets "trail" on the film.
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Planet-trail photo! For example, if we take a time exposure of Saturn and Jupiter - we would see that there is an apparent separation in the sky of their trails. Here is a 10 minute exposure of two planets : Saturn (on the left) and Jupiter (on the right). [Notice the Pleiades - a cluster of stars - on the far left - they form a tight group of trails!] It seems like we need to know more about the Equator and the Ecliptic to understand what is really happening in this photo! Keep reading! |
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How is the ecliptic oriented relative to the equator? In this illustration we pretend the solar system is horizontal (the usual way we think of it). Since the Earth axis is tilted about 23 degrees away from the Solar system plane, the Earth's Equator will tilt up by that same 23 degrees as illustrated. There are two places where the curves cross - each are the "Equinoxes" - in the Spring is the Vernal Equinox - in the Fall is the Autumnal Equinox. (In fact, if we want to talk about where planets are relative to each other, we need a way to measure the angles from the Sun. The "reference direction" chosen is the line that connects the Earth and the Sun at the moment of the Spring Equinox {where the two curves to the left intesect}. Continue that line through the Sun and out the other side, and it becomes the line in space that we start the angles from. |
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Where are Jupiter and Saturn currently? Here is a plot that shows the positions of Jupiter and Saturn around their orbits (roughly) and shows where the Earth is in its orbit. [Note, this also explains why we see Jupiter to the "West" of Saturn in the sky - Saturn is farther along the orbit than Jupiter - in fact, in a few years, since Jupiter is closer than Saturn, and moves faster in the orbit - Jupiter will "line up" with Saturn, and then pass Saturn, and be "East" of Saturn for a few years, and then it will "catch up" to Saturn again from the "West". {Jupiter has an orbit period of about 12 years, Saturn has an orbit period of about 30 years.}] Also, notice the lines drawn from "the sun" to the planets - the line for the Earth is almost pointing completely to the right. Near the end of September, the Earth will be directly to the right of the Sun in this diagram (6 months later, it will be directly to the left - the Spring Equinox!). This illustration was made with HomePlanet - see info in "references" section below. |
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Let's connect that back to the photo of Jupiter and Saturn! Look back at the planet-trail photo. Notice the red lines (they were added afterwards .. the planets don't leave colored streaks like that - ha!) - they seem to indicate that Saturn and Jupiter are not trailing along the same line. But, this is misleading!!! Saturn and Jupiter are slightly above the ecliptic (we know that from the previous picture) - but, because of the angle between the ecliptic and the equator, it makes the trails for the planets appear to be separated! So any of the planets we try to "trail-photograph" will trail at an angle relative to their actual path through the Solar system. |
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Curving planet trails? If you have sharp eyes, you will notice in the planet-trail photo above, the stars near the planets (and the planets themselves) appear to be bending "upward" a little (the North Star is in the direction of the upper left corner of that picture. That is another indication that the curves relate to the equator ... the ecliptic is "above" the equator (closer to the North Star) in that part of the sky .. so all the trails of the planets in that part of the sky will curve upwards!
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Is there a constellation that "straddles" the equator? Yes, the winter constellation Orion (the one with the three stars in the belt) overlaps the equator. If you took a long-time exposure picture of Orion, you might be able to see the stars in the upper part curve upwards, and the stars in the lower part curve downwards - I'll try that this winter and post the results here! (Here is a figure from HomePlanet for Orion {I added the stars so we could see it better}. The belt for Orion is just below the equator - with very long time exposures, it might be possible to see the top part of Orion trying to curve to the North, and the bottom trying to curve to the South! |
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Star Trails in Orion? Since the constellation straddles the equator, the stars within it will basically travel in straight lines with our star trail pictures. (If we have a long enough exposure, we might see the stars to the "north" trail with slight upward curves, and the stars to the "south" trail with slightly downward trails. Here is a picture taken very early in the morning in September (Orion is high in the sky in the winter .. but starts rising in the fall way past midnight). I need a darker sky, to be able to get the contrast better - but you can see the nearly straight lines. [Click on the image to see a larger version.] |
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References :
All of the photographs are my own. Some of the illustrations here were created by me personally. The HomePlanet illustrations were created using a fabulous Astronomy Software program called HomePlanet. (The link contains information on HomePlanet 2 which would run on Windows 3.1 and Home Planet 3 which is for 32 bit Windows {9x or NT}. I highly recommend this program!!)