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

December 4, 2007 by Fred Bortz

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!