Happy Uranian Equinox, a once-in-a-half-lifetime event

The last time Uranus experienced an equinox, planetary astronomer Heidi Hammel was in Kindergarten.

This time, at age 47, she is at the forefront of discoveries about its atmosphere and climate.

The reason Uranus’ equinox is of such importance to Dr. Hammel and her fellow planetary astronomers is the planet’s unusual inclination, 98 degrees, which means that its poles point nearly directly toward or away from the Sun at solstice, giving Uranus the most extreme seasonal changes in solar illumination by far among the other worlds of the Solar System.

Dr. Hammel has actually been studying the Uranian equinox for the past few years. The exact date is December 7, 2007, but the planet’s long orbital period–about 84 years–means that it has been essentially in equinox for many months. Changes that take place on in a day on Earth take nearly three Earth months to unfold on Uranus. It’s been the climatic equivalent of mid-September on Uranus since 2005.

The most dramatic visual effects of the equinox period, such as having the rings of Uranus edge-on to the sun and capturing a shadow of one of its moons moving across its face, have only been observed this year. (See my Heidi Hammel update page for examples.)

According to my most recent e-mail exchange with Dr. Hammel, she expects to be observing at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea during the Uranian equinox. But she won’t be observing Uranus.

[CORRECTION: She will be observing Uranus, even though the time is not ideal. I will leave the rest of the post unchanged, because it has some useful educational material, but please see the full correction in the comment below.]

Why? Because prime time for observing a planet comes at its “opposition,” when it is most directly opposite the Sun as viewed from Earth. For Uranus, that was September 10 this year. Three months later, it is much farther from Earth and a good bit lower in the sky, which means it is harder to see and is viewable for much less time on any given night.

So Dr. Hammel is observing a planet which has not been a specialty, Mars. Mars’ opposition is December 24, but it is already approaching prime viewing position. She and her colleagues will especially be studying Mars’ two tiny moons, Deimos and Phobos, just as they did in the off moments when I was with them in 2003. That observing session produced surprising results about the moons’ infrared spectrum and justified another look.

My young readers’ biography of Dr. Hammel, Beyond Jupiter, includes a description of that session. I, for one, will be eagerly awaiting word about follow-up discoveries from this observation.

To my friends at IRTF for this observing trip, I wish you clear skies, problem-free equipment, pleasant conversation and music from “The Big Kahuna” radio station, and a happy Uranian Equinox!

But most of all, I wish you wonderful discoveries and questions about Deimos, Phobos, and Mars!

3 thoughts on “Happy Uranian Equinox, a once-in-a-half-lifetime event”

  1. It seems that Heidi Hammel will be observing Uranus at Equinox after all, leaving the observation of Mars to others (“the BASS guys,” who are pictured in the book and the webpage about the 2003 observation). She’ll be using the Keck telescope, which appears in some of the 2003 images.

    A number of other major telescopes on Earth and in orbit will be observing the event as well. The Spitzer Space Telescope is an infrared instrument in a solar orbit that trails the Earth. It was launched in the same week in 2003 that I joined Heidi and the BASS guys on Mauna Kea.

    Here’s her latest e-mail to me.

    Hi Fred,

    Yes, some corrections/additions to the blog:

    Actually I am (almost) to Hawaii, but not to the IRTF.

    I am in California, enroute to the Keck Observatory, where we *ARE* in fact observing the equinox of Uranus with Keck’s fantastic adaptive optics imaging system.

    We only have the first few hours of the night, since that’s all the time that Uranus is up. After it sets, we turn the telescope over to others.

    The BASS guys are covering Mars from the IRTF on their own.

    We also have time on the Spitzer Space Telescope for Uranus equinox mid-infrared spectral observations – probably next week. Hubble cannot observe Uranus right now just due to the geometry, too close to the sun (“solar avoidance”).

    We also hope the Gemini Telescope will get some mid-infrared images this week or next.

    Hope that is helpful.


    Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)

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