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Emily LakdawallaDecember 7, 2007

It's the first day of spring on the northern hemisphere of Uranus

Happy Equinox to our Uranian colleagues! Today is the day that Uranus' north pole is seeing the Sun for the first time in 42 years; for astronomers, the equinox is more precisely defined as when the sub-solar point on the planet is at the equator. Remember that Uranus is the planet tilted almost exactly sideways, with an axial tilt of 98 degrees; so it's a big deal when the seasons change. Of course, the change of the seasons is quite gradual, and the atmosphere of Uranus has been shifting and changing a lot in recent years as Sunlight gradually crawls northward, something I've written about before.

Outer planets astronomers are in the thick of a busy schedule of observing Uranus during this dynamic time; there are few astronomers who will be privileged to see this event unfold twice in their professional lifetimes. They're studying how sunlight forces weather activity in the atmosphere and how the northern and southern hemispheres differ (and whether those differences, from before the equinox to after, flip from one hemisphere to the other); they're watching the rings, particularly as Earth crosses the ring plane, which happened twice in the last year and will happen one more time, on February 20; they're studying mutual events of Uranus' moons, when one satellite crosses another; and lots of other stuff.

IR Keck images of Uranus ring crossing

Imke de Pater (UC Berkeley), Heidi B. Hammel (SSI, Boulder) and the W. M. Keck Observatory

IR Keck images of Uranus ring crossing
The sun-lit side of the rings of Uranus (two left images) captured in the infrared by the Keck II telescope in 2004 and 2006 as Uranus approached the point where its rings appear edge-on from Earth. On May 28, 2007, less than a month after the first ring-plane crossing on May 3, Keck II captured the unlit side of the rings for the first time ever (right). The dotted lines show the position of the epsilon (upper line) and zeta (lower line) rings, with other rings indicated.
Of course, the best time to observe any solar system object is during its opposition, when Earth is directly between the sun and the object of interest. At opposition, an object is closer to Earth than at any other time. This isn't too important for distant outer planets because the distance doesn't vary much over the course of the year. What's more important is that during opposition, an object is at its maximum distance from the Sun in the sky and is up in the night sky for a maximum length of time, making observing much easier. Uranus' opposition was on September 10, almost three months ago, so now isn't the greatest time to view it, and according to this blog the weather isn't cooperating at Mauna Kea anyway.

Anyway, I invite you to start a conversation this weekend by mentioning the title of today's blog entry in conversation. "Uranus" jokes may be cheap but I've come to terms with the fact that they are a reliable method for breaking through the apathy of unenlightened folks who don't care about space.

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Emily Lakdawalla

Solar System Specialist for The Planetary Society
Read more articles by Emily Lakdawalla

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