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November 2019

Current Events: Moonrise at Sunrise

Credit: NASA/Christina Koch

By Stacy Palen

The image above was taken from the International Space Station by NASA astronaut Christina Hammock Koch and could form the basis for a nice final exam question.


1) In this image, what is the phase of the Moon? How do you know?

The moon here is a Waning Crescent. The image was taken looking East, into the sunrise, so north is to the left in this image. Therefore, the Moon is “leading” the Sun across the sky and illuminated on the eastern half. This means it is in the Waning phases.


2) If you had only this image to go by, with no caption, how would you know that it was an image of Earth?

The planet shown has a solid surface and a shallow but significant atmosphere. Earth is the only terrestrial planet that fits these criteria.

Other potential answers:

  • It could not be Venus or Mercury because neither planet has a moon.
  • Mars’ atmosphere is too thin to refract blue light in this way.
  • There appears, possibly, to be a hint of colors other than red on the surface.


3) Based only upon the image, would you think this planet might be a good place to look for life? What other information could you gather from this viewpoint that would make you more certain of your conclusion?

This planet may be a good place to look for life. The solid surface and the presence of an atmosphere would make me want to investigate further. I would especially want to get a closer look at the surface, or get a spectrum of the atmosphere in order to be totally sure.

Reading Astronomy News: ESO Telescope Reveals What Could be the Smallest Dwarf Planet yet in the Solar System

ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS)

By Stacy Palen

According to this article by the ESO, Hygiea has been imaged by ESO’s VLT. It is round, which makes it a dwarf planet rather than an asteroid. Smaller than Ceres, it is now the smallest known dwarf planet in the Solar System.


1) Study the image of Hygiea at the top of the article. The image of this tiny object is a little bit fuzzy, despite the powerful telescope used to obtain it. Nonetheless, some features are visible. Describe the surface of Hygiea.

Hygiea appears to have some craters, with variations in height as well as variations in brightness.


2) Until this image was taken, astronomers were not sure whether to categorize Hygiea as an asteroid or a dwarf planet. Which criterion for dwarf planet status could be determined from this image?

Hygiea has enough gravity to pull itself into a round shape.


3) The article compares Hygiea’s size (430 km) to that of both Pluto (2400 km) and Ceres (950 km). Roughly how many times larger are these other dwarf planets than Hygiea?

Ceres is a bit more than twice as large, and Pluto is about six times larger.


4) Describe the origin of this dwarf planet, in your own words.

A much larger planetesimal collided with a smaller one about 2 billion years ago. The explosion created 7,000 asteroids, at least one of which had enough gravity to form a dwarf planet.

Classroom Stories: Vera Rubin Tells The Story

Dark Matter
Image credit: NASA/JPL-Caltech/ESA/Institute of Astrophysics of Andalusia, University of Basque Country/JHU

By Stacy Palen

I was poking around, looking for something completely different when I came across this nice little vignette from "Physics Today" published in 20061. It’s the story of the discovery of dark matter, told by Vera Rubin herself.

The story is mostly accessible to introductory students, with only a little bit of stretch required in the single paragraph that describes circular velocities and flat rotation curves. Hilariously, she includes an "exercise for the reader.” (Well—hilarious to me, and probably you, but students won’t get it.)

If your students have already learned about galaxy rotation curves, they will be able to follow the paragraph. If not, it’s fine if they skim over it—they won’t lose the plot.

The descriptions of observing at the telescope, and the trouble of moving the spectrograph from one location to another really gives a nice feel for how hard it was to get this done the first time.

I’m not entirely sure what I’m going to do with this story in my classes yet, but I found it charming, and think it will capture the interest of some of my students who struggle to connect to this material. I’ll at least share it with them through the LMS so that students who are interested can read it.

If you come up with a plan to use it, tell me about it in the comments!


1 Unfortunately, the biographical information published with the article is out of date. Vera Rubin passed away at the end of 2016.

Current Events: Naming the Moons of Saturn

By Stacy Palen

I long ago stopped keeping track of the number of moons around Saturn and Jupiter. It often feels like there is a contest going on among astronomers—who can find the most moons around “their” planet! In early October, a report hit the news of 20 new moons discovered around Saturn, many of them in retrograde orbits.

This brings the number of moons around Saturn to 82. The number around Jupiter? Only 79. Neener-neener, Jupiter devotees!

Seriously though, if you are now discussing moons, planets, or planetary formation, this is a timely discovery to talk about with students. Or if you are about to start talking about dark matter, it may be a good time to remind students about Keplerian orbits and Newton’s version of Kepler’s Third Law.

But wait! There’s more!

These moons are not yet named, and Scott Sheppard has decided to have a contest to name them all.

Here are the general rules from the Carnegie Science website:

  • Two of the newly discovered prograde moons fit into a group of outer moons with inclinations of about 46 degrees called the Inuit group. All name submissions for this group must be giants from Inuit mythology.
  • Seventeen of the newly discovered moons are retrograde moons in the Norse group. All name submissions for this group must be giants from Norse mythology.
  • One of the newly discovered moons orbits in the prograde direction and has an inclination near 36 degrees, which is similar to those in the Gallic group, although it is much farther away from Saturn than any other prograde moons. It must be named after a giant from Gallic mythology.

Full details are available on the website along with a link to a list of names already used and a little video that describes the contest.

It would be great fun to make a class project or competition (even for college students) to choose a name to submit as a group. It’s the kind of experience that students remember for a long time. Submissions are due via Twitter by December 6.