Understanding Conjunctions

00:00 In the previous lesson, I gave you an overview of the course. In this lesson, I’ll be staying within our solar system and looking for planetary conjunctions.

00:09 A planetary conjunction is when two or more of them line up in the sky. This happens when the viewing angle between them is small.

00:17 Consider a star with three planets around it. These two planets aren’t in conjunction as their angle of separation is wide, but the first and third planets are as the angle of separation is narrow.

00:34 You could argue that true conjunction would be an angle of separation of zero, but to keep the computation low, I’m going to use a slightly larger value than that.

00:43 How do you find planetary conjunctions? Well, looping through every planet and comparing it to every other planet for any given day, and then doing it for a lot of days is computationally expensive.

00:54 To reduce the amount of computing to be done, you can take advantage of the fact that Mercury is a speedy little guy, and in fact, that’s why it’s called Mercury.

01:03 The Romans named it after their fleet-footed winged, actually messenger god. Mercury the planet, not the god, has an orbital period of 88 days. Calculating angular separation is done between two bodies, which means each calculation needs a reference and a target.

01:21 Using Mercury as a reference point against all the planets is helpful. Those planets further out have slower angular velocities because their orbital distance is so much larger.

01:32 If two of the further out planets are in conjunction, then in all likelihood, they still will be the next time Mercury swings around. This isn’t perfect, but for our purposes, it gets us some answers and means far less calculation.

01:47 To calculate the angular separation between two bodies in the sky, you need to know where they are. There are actually several different coordinate systems that astronomers use.

01:58 In this lesson, I’ll be using right ascension and declination. This is kind of like longitude and latitude, but using angles from the equator.

02:08 And just how do you get the coordinates of an object in the sky or calculate their angular separation? Well, there’s a library for that. Astropy is a third-party library containing all sorts of tools for astronomy.

02:21 It includes a database of stars and planetary bodies, and has an entire module dedicated to coordinate systems. The SkyCoord object contains the right ascension and declination of an object.

02:35 Since Astropy is a third-party library, that means you need to install it using pip. As always, don’t forget to use a virtual environment.