Predicting planetary visibility with PyEphem (Shallow Thoughts)

Predicting planetary visibility with PyEphem

Part 1: Basic Planetary Visibility

All through the years I was writing the planet observing column for the San Jose Astronomical Association, I was annoyed at the lack of places to go to find out about upcoming events like conjunctions, when two or more planets are close together in the sky. It's easy to find out about conjunctions in the next month, but not so easy to find sites that will tell you several months in advance, like you need if you're writing for a print publication (even a club newsletter).

For some reason I never thought about trying to calculate it myself. I just assumed it would be hard, and wanted a source that could spoon-feed me the predictions.

The best source I know of is the RASC Observer's Handbook, which I faithfully bought every year and checked each month so I could enter that month's events by hand. Except for January and February, when I didn't have the next year's handbook yet by the time my column went to press and I was on my own. I have to confess, I was happy to get away from that aspect of the column when I moved.

In my new town, I've been helping the local nature center with their website. They had some great pages already, like a What's Blooming Now? page that keeps track of which flowers are blooming now and only shows the current ones. I've been helping them extend it by adding features like showing only flowers of a particular color, separating the data into CSV databases so it's easier to add new flowers or butterflies, and so forth. Eventually we hope to build similar databases of birds, reptiles and amphibians.

And recently someone suggested that their astronomy page could use some help. Indeed it could -- it hadn't been updated in about five years. So we got to work looking for a source of upcoming astronomy events we could use as a data source for the page, and we found sources for a few things, like moon phases and eclipses, but not much.

Someone asked about planetary conjunctions, and remembering how I'd always struggled to find that data, especially in months when I didn't have the RASC handbook yet, I got to wondering about calculating it myself. Obviously it's possible to calculate when a planet will be visible, or whether two planets are close to each other in the sky. And I've done some programming with PyEphem before, and found it fairly easy to use. How hard could it be?

Note: this article covers only the basic problem of predicting when a planet will be visible in the evening. A followup article will discuss the harder problem of conjunctions.

Calculating planet visibility with PyEphem

The first step was figuring out when planets were up. That was straightforward. Make a list of the easily visible planets (remember, this is for a nature center, so people using the page aren't expected to have telescopes):

import ephem

planets = [
    ephem.Moon(),
    ephem.Mercury(),
    ephem.Venus(),
    ephem.Mars(),
    ephem.Jupiter(),
    ephem.Saturn()
    ]

Then we need an observer with the right latitude, longitude and elevation. Elevation is apparently in meters, though they never bother to mention that in the PyEphem documentation:

observer = ephem.Observer()
observer.name = "Los Alamos"
observer.lon = '-106.2978'
observer.lat = '35.8911'
observer.elevation = 2286  # meters, though the docs don't actually say

Then we loop over the date range for which we want predictions. For a given date d, we're going to need to know the time of sunset, because we want to know which planets will still be up after nightfall.

observer.date = d
sunset = observer.previous_setting(sun)

Then we need to loop over planets and figure out which ones are visible. It seems like a reasonable first approach to declare that any planet that's visible after sunset and before midnight is worth mentioning.

Now, PyEphem can tell you directly the rising and setting times of a planet on a given day. But I found it simplified the code if I just checked the planet's altitude at sunset and again at midnight. If either one of them is "high enough", then the planet is visible that night. (Fortunately, here in the mid latitudes we don't have to worry that a planet will rise after sunset and then set again before midnight. If we were closer to the arctic or antarctic circles, that would be a concern in some seasons.)

min_alt = 10. * math.pi / 180.
for planet in planets:
    observer.date = sunset
    planet.compute(observer)
    if planet.alt > min_alt:
        print planet.name, "is already up at sunset"

Easy enough for sunset. But how do we set the date to midnight on that same night? That turns out to be a bit tricky with PyEphem's date class. Here's what I came up with:

    midnight = list(observer.date.tuple())
    midnight[3:6] = [7, 0, 0]
    observer.date = ephem.date(tuple(midnight))
    planet.compute(observer)
    if planet.alt > min_alt:
        print planet.name, "will rise before midnight"

What's that 7 there? That's Greenwich Mean Time when it's midnight in our time zone. It's hardwired because this is for a web site meant for locals. Obviously, for a more general program, you should get the time zone from the computer and add accordingly, and you should also be smarter about daylight savings time and such. The PyEphem documentation, fortunately, gives you tips on how to deal with time zones. (In practice, though, the rise and set times of planets on a given day doesn't change much with time zone.)

And now you have your predictions of which planets will be visible on a given date. The rest is just a matter of writing it out into your chosen database format.

In the next article, I'll cover planetary and lunar conjunctions -- which were superficially very simple, but turned out to have some tricks that made the programming harder than I expected.