Astronomy with a cellphone — planetary orbits
Saturn is undergoing apparent retrograde motion this month (Aug ‘22).
If you’ve been reading the other Astronomy with a cellphone articles they have been more about the general use of cellphones for astronomy and not a specific astronomy topic. With this post we’ll actually do something astronomical: record the retrograde motion of Saturn.
The English word planet traces back to the Greek word πλανήτης which became the Latin word planeta. It’s derived from the verb “to wander,” and planets were sometimes called the wandering stars.
We now know that planets are not stars, but are objects in our solar system that are visible because they reflect light from our local star, the Sun. The other points of light we see in the night sky are other suns, the closest of which is still 10s of thousands times farther away than even the furthest planet we see.
If you could freeze Earth in its orbit around the sun and watch the planets Mars, Jupiter, or Saturn you would see that each night they are a little further East of their location than the night before. This would also apply to Uranus, Neptune, and anything else with an orbit outside of Earth’s orbit that goes around the Sun the same way; the exception being some comets. For simplicity we’ll just stick to the naked eye visible planets in our discussion.
Of course if the Earth didn’t orbit the Sun it would be very bad as then we would fall in to the sun in about 2 months. Knowing how orbits work could help reassure us that we’re not all going to be burnt toast soon, and observing other planet’s orbits would be a good first step in understanding orbits.
Since the Earth is not frozen in one location we actually observe that the outer planets spend part of the year moving across the sky east to west, and some amount of time in the opposite direction, west to east. Of course in real life planets don’t stop and reverse direction in their orbits as that at a minimum would violate Newton’s theories of motion. This is why the expression used to describe this is called apparent retrograde motion. The planets are not actually changing how they move.
Rather than repeat the details about why the outer planets exhibit apparent retrograde motion when observed from Earth I’ll just point you to the Wikipedia page on the topic, which also includes a number of animations to help explain it.
Turning these observations in to orbital parameters literally fills a book on the topic of Celestial Mechanics. While the math is mostly at high school trig level, the details are rather involved. For a nice open source read on the topic see the book by Tatum.
You can cheat and use desktop planetarium software to plot where a planet will be each night. The (free) program SkyChart / Cartes du Ciel is great for this, as shown in the example below for Saturn for August through December of this year (2022).
Armed with nothing but your cell phone and a couple of clear nights you can document the retrograde motion of Saturn. For Northern hemisphere observers Saturn will appear low in the southeast in August. It’s fairly easy to spot as it’s brighter than the surrounding stars.
If you can, try and take a picture every few nights with Saturn in the same position, i.e. nominally around the same time each night.
It’s tough to see the motion by just looking at successive pictures. Over a few nights, without zooming in on the pictures, you might not even notice the motion.
To make the motion more obvious you want to create a short sequence of each of the pictures and show each one for 2 or 3 seconds. There are a lot of ways to create an animation from a series of still photos. For this example Powerpoint was used as it can directly export a movie file. Animated .gif files do not always play as expected on some devices/browsers so a movie file is more likely to work across platforms.
The steps were:
- Crop each photo to the region of interest. This makes it easier to work with later.
- Place each photo on its own page of a Powerpoint document.
- Scale the images to fill the page. It’s important to use the same scale factor, so don’t do this by dragging the image handles. Instead set it in the image properties.
- Place guides on a pair of stars to determine orientation. Rotate each picture to line up the same.
- If needed add background fill if the picture doesn’t fill the page after rotation. Alternately start with more picture to ensure it covers the page.
- Adjust brightness/contrast of the photos to reasonably match each other. In an ideal world this wouldn’t be needed, but clouds, moonlight, etc., can lead to different brightness of the sky background.
- (optional) Add the date to each slide
- Export the deck as a movie
Despite the length of this list it takes just a few minutes to do.
Doing this for the four pictures taken results in this final animation of Saturn’s apparent retrograde motion.
This method isn’t perfect; there is some camera distortion that results in stars near the edge appearing to shift. Without a permanent fixed mount for the phone and placing Saturn in the same exact spot in the field this type of thing is unavoidable.
The last picture was taken on a night with some clouds and the moon was out, which washed out a lot of the stars. Unless you live in a cloud free desert climate differences in the pictures will need to be adjusted for.
Powerpoint is flexible enough to get the images matched enough so you can focus on the object of interest. This same process was used to create an animation of Mars, though here just two photos were available and is not as a nice result as the 4 image sequence of Saturn.
