I have another PEEC Planetarium talk coming up in a few weeks,
a talk on the
co-presenting with Chick Keller on Fri, Jun 18 at 7pm MDT.
I'm letting Chick do most of the talking about archaeoastronomy
since he knows a lot more about it than I do, while I'll be talking
about the celestial dynamics -- what is a solstice, what is the sun
doing in our sky and why would you care, and some weirdnesses relating
to sunrise and sunset times and the length of the day.
And of course I'll be talking about the analemma, because
just try to stop me talking about analemmas whenever the topic
of the sun's motion comes up.
But besides the analemma, I need a lot of graphics of the earth
showing the terminator, the dividing line between day and night.
Monday was the last night it's been clear enough to see Comet Neowise.
I shot some photos with the Rebel, but I haven't quite figured out
the alignment and stacking needed for decent astrophotos, so I don't
have much to show. I can't even see the ion tail.
The interesting thing about Monday besides just getting to see
the comet was the never-ending train of satellites.
Comet C/2020 F3 NEOWISE continues to improve, and as of Tuesday night
it has moved into the evening sky (while also still being visible in
the morning for a few more days).
I caught it Tuesday night at 9:30 pm. The sky was still a bit bright,
and although the comet was easy in binoculars, it was a struggle to see
it with the unaided eye. However, over the next fifteen minutes the sky
darkened, and it looked pretty good by 9:50, considering the partly
cloudy sky. I didn't attempt a photograph; this photo is from Sunday morning,
in twilight and with a bright moon.
I've learned not to get excited when I read about a new comet. They're
so often a disappointment. That goes double for comets in the morning
sky: I need a darned good reason to get up before dawn.
But the chatter among astronomers about the current comet, C2020 F3
NEOWISE, has been different. So when I found myself awake at 4 am,
I grabbed some binoculars and went out on the deck to look.
And I was glad I did. NEOWISE is by far the best comet I've seen
since Hale-Bopp. Which is not to say it's in Hale-Bopp's class --
certainly not. But it's easily visible to the unaided eye, with a
substantial several-degree-long tail. Even in dawn twilight. Even
with a bright moon. It's beautiful!
Update: the morning after I wrote that,
get a photo,
though it's not nearly as good as Dbot3000's that's shown here.
When I was in grade school -- probably some time around 7th grade -- I
happened upon an article in Scientific American about the Anasazi Sun
Dagger on Fajada
Butte in Chaco Canyon. On the solstices and equinoxes, a thin
dagger of light is positioned just right so that it moves across a
spiral that's carved into the rock.
I was captivated. What an amazing sight it must be, I thought.
I wondered if ordinary people were allowed to go see it.
Well, by the time I was old enough to do my own traveling, the answer
was pretty much no. Too many people were visiting Fajada Butte ...
Galen Gisler, our master of Planetarium Tricks,
presented something strange and cool in his planetarium show last Friday.
He'd been looking for a way to visualize
the "Venus Pentagram", a regularity where Venus'
inferior conjunctions -- the point where Venus is approximately
between Earth and the Sun -- follow a cycle of five.
If you plot the conjunction positions, you'll see a pentagram,
and the sixth conjunction will be almost (but not quite) in the
same place where the first one was.
Supposedly many ancient civilizations supposedly knew about this
pattern, though as Galen noted (and I'd also noticed when researching
my Stonehenge talk), the evidence is sometimes spotty.
Galen's latest trick: he moved the planetarium's observer location
up above the Earth's north ecliptic pole. Then he told the planetarium to
looked back at the Earth and lock the observer's position so it
moves along with the Earth; then he let the planets move in fast-forward,
leaving trails so their motions were plotted.
The result was fascinating to watch. You could see the Venus pentagram
easily as it made its five loops toward Earth, and the loops of all
the other planets as their distance from Earth changed over the course
of both Earth's orbits and theirs.
You can see the patterns they make at right, with the Venus pentagram
marked (click on the image for a larger version).
Venus' orbit is white, Mercury is yellow, Mars is red.
If you're wondering why Venus' orbit seems to go inside Mercury's,
remember: this is a geocentric model, so it's plotting distance from
Earth, and Venus gets both closer to and farther from Earth than Mercury does.
He said he'd shown this to the high school astronomy club and their
reaction was, "My, this is complicated." Indeed.
It gives insight into what a difficult problem geocentric astronomers
had in trying to model planetary motion, with their epicycles and
Of course that made me want one of my own. It's neat to watch it in
the planetarium, but you can't do that every day.
So: Python, Gtk/Cairo, and PyEphem. It's pretty simple, really.
The goal is to plot planet positions as viewed from high
above the north ecliptic pole: so for each time step, for each planet,
compute its right ascension and distance (declination doesn't matter)
and convert that to rectangular coordinates. Then draw a colored line
from the planet's last X, Y position to the new one. Save all the
coordinates in case the window needs to redraw.
At first I tried using Skyfield, the Python library which is supposed
to replace PyEphem (written by the same author). But Skyfield, while
it's probably more accurate, is much harder to use than PyEphem.
It uses SPICE kernels
(my blog post
on SPICE, some SPICE
examples and notes), which means there's no clear documentation or
list of which kernels cover what. I tried the kernels mentioned in the
Skyfield documentation, and after running for a while the program
died with an error saying its model for Jupiter in the de421.bsp kernel
wasn't good beyond 2471184.5 (October 9 2053).
Rather than spend half a day searching for other SPICE kernels,
I gave up on Skyfield and rewrote the program to use PyEphem,
which worked beautifully and amazed me with how much faster it was: I
had to rewrite my GTK code to use a timer just to slow it down to
where I could see the orbits as they developed!
It's fun to watch; maybe not quite as spacey as Galen's full-dome view
in the planetarium, but a lot more convenient.
You need Python 3, PyEphem and the usual GTK3 introspection modules;
on Debian-based systems I think the python3-gi-cairo package
will pull in most of them as dependencies.
I'm jazzed about this show. I think it'll be the most fun
planetarium show we've given so far.
We'll be showing a variety of lunarfeatures:
maria, craters, mountains, rilles, domes, catenae and more.
For each one, we'll discuss what the feature actually is and how it
was created, where to see good examples on the moon,
and -- the important part -- where you can go on Earth,
and specifically in the Western US,
to see a similar feature up close.
Plus: a short flyover of some of the major features using the
full-dome planetarium. Some features, like Tycho, the
Straight Wall, Reiner Gamma, plus lots of rilles, look really great
in the planetarium.
If you can't get to the moon yourself,
this is the next best thing!
The Hitchhiker's Guide to the Moon:
7pm at the PEEC nature center. Admission is free.
Come find out how to explore the moon without leaving your home planet!