After my last astronomy post, I was curious if I could develop a cosmic sense of direction, to know, day or night, where Sirius is the same way I generally know which way is north.
So I built this interactive sidereal clock. I wil explain a little bit about how it works and then how I got there.
(This is a screenshot. Click through to see the real version.)
The clock displays both solar time (24 hours per day) and sidereal time (four fewer minutes per day, but measured as 24 sidereal hours). The outside circle shows the sidereal hours, and the inner circle (representing Earth) shows the solar hours.
Just like 12:00 solar time is when the Sun is at its peak, 6:00 sidereal time is when Betelgeuse in Orion is at its peak. Because stars are so far away, they don’t seem to move much relative to each other, so every (non-polar) star has a fixed sidereal time when it reaches its peak. This is also called right ascension, abbreviated RA.
The right ascension is essentially a measure of longitude, and at 6:00, the 6ʰ line runs from Polaris, straight across the highest point of the sky and down to the southern horizon, where out of sight from the northern hemisphere it hits the south celestial pole. At the same time, the 12ʰ line goes from Polaris down to the eastern horizon, and the 0ʰ line to the western horizon. Also at that time, a small part of the 18ʰ line is visible in the northern hemisphere, from Polaris straight to the northern horizon.
In the interactive clock, you can drag the Sun around to represent different times of year. This shows you the relative position of the Sun, Earth, and stars at that time of year. You can see that the winter solstice is when the Earth is between the Sun and Orion. In other words, the northern hemisphere is always, regardless of the season, tilted towards Orion. (That won’t last forever, but it will for my lifetime.) I marked the four seasons on the clock, and when the Sun circles a season, that represents the solstice or equinox (again, in the northern hemisphere).
You can also drag the little human standing on Earth to adjust the time of day. The perspective of the Earth is looking down from above the north pole. It is easiest to imagine yourself standing on Earth looking south, with the eastern horizon on your left and the western horizon on your right. Although, again, to see the entire line corresponding to the current sidereal time, you will have to look up, up, up, and a little backwards, until you see Polaris.
You can see the difference between sidereal and solar time, because if you drag the guy once around, the Sun will have moved slightly so you have to drag a bit further to complete a solar day.
The effect of the Earth’s tilt on the Sun’s position is not shown here. The Sun’s RA is correct, though. The relative movement of the Sun above or below the Earth’s equator is what causes it to rise somewhat before or after 6am, depending on your latitude and the season.
Although I’ve never had a very good grasp on astronomy, it is not for lack of experience. When I was young, my mom was very interested in astronomy, and we used to drive out to the potato fields of Idaho to see the night sky. We had a Planisphere. We had star mapping software, probably DOS-based, but I don’t remember which. But everything, to me, always felt like it was moving, and never in a way I could understand or visualize. (I think most astronomy software is especially confusing in this respect.)
For this reason, I really wanted something that let me imagine the stars holding still, while the Earth and Sun moved. As far as I know, there isn’t a “standard” way to depict the night sky, but to my math brain it made sense to put 0 on the positive x axis. A top-down view allows you to have the cardinal directions in their normal locations, although it prioritizes the southern sky and makes the clock run counterclockwise. At my latitude, the equatorial and mid-southern cosntellations make up more of the sky anyway. If I still lived in Idaho or the Pacific Northwest, it might make more sense to flip things around and look up at the earth from below, since the polar constellations are so much more prominent and probably an easier way to orient yourself.
I used Javascript (mostly d3) to build an SVG file. At first I had sliders for the time of year and time of day, and then I realized I could just watch for drag events on certain parts of the image. Feel free to take a look at the code, it is only a few hundred lines of very spaced out code.
I used a Planisphere to make sure that I was getting things correct.
I had a ton of fun making this. The process of working everything out really helped it click into my brain.
Ultimately, the easiest way to orient myself seems to be to think first about where the Sun is in relation to the stars, and then where I am in relation to the Sun, rather than think explicitly about sidereal time. Which is essentially how it has been done for thousands of years with the Zodiac, but I find that the extra step of fixing the Earth’s orbit as up-down-left-right is critical for my spatial awareness.