December 1 – ISON’s first

Today’s factismal: Most comets are at their brightest just before dawn or just after dark.

As you have probably heard, the comet ISON (named for the International Scientific Optical Network that found it) has passed its closest approach to the Sun (known as perihelion to the astronomy wonks and Too Darn Hot to the Cole Porter aficionados) and is headed back out to the depths of the Solar System where it was born. Right now, it appears that ISON mostly survived the encounter, though it may have tossed off a few baby comets thanks to the extreme gravitational stresses on its fragile body. (Like most comets, ISON is a dirty snowball of mostly frozen ices and a healthy dusting of rock grains.) If you would like to see ISON, the best time is in the morning just before sunrise; look to the East, about a hand and a half up from the horizon.

Comet ISON's trip around the Sun as seen by SOHO (Image courtesy NASA)

Comet ISON’s trip around the Sun as seen by SOHO
(Image courtesy NASA)

But why in the world do we have to get up so early to see comets? The answer, my friend, is blowing in the solar wind. As the comet heads into the inner Solar System, it is till cold and mostly frozen. Though some of the ices do warm up and turn to gas and escape from the comet to form its two tails (a curved one consisting of dust and straight one pointing away from the Sun one composed of  ionized gasses) before the comet gets to the Sun, most of the changes take place afterwards thanks to the physical phenomenon known as “thermal lag”. Anyone who ha made tea knows about thermal lag; it is the time between when you put the kettle on to boil and when it does boil – and how it keeps boiling for a bit after you’ve taken the kettle off.

The two tails of Hale-Bopp (Image courtesy NASA)

The two tails of Hale-Bopp
(Image courtesy NASA)

So, because the comet only heats up as it nears the Sun and because most of the outgassing takes place after it has passed the Sun, most comets can only be seen when they are very near the Sun. And that means looking for them just before sunrise or just after sunset. If you’d like to learn more about comets like ISON and maybe report a comet or two of your own, then head on over to the SOHO Sungrazer site:
http://sungrazer.nrl.navy.mil/

December 2 update:
ISON appears to be well and truly dead. For more details, see the obituary by the NASA Comet ISON Observing Campaign:
http://www.isoncampaign.org/karl/in-memoriam

On the bright side, we can expect to see a great meteor shower when the Earth passes through the remains of comet ISON in mid-January.

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