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What's in a Name?

We prefer to make our comets more cozy by using their common names: Comet McNaught, Comet Thatcher, Halley's Comet, the Great Comet of 1811 ... But astronomers follow a different set of rules when referring to these icy travelers. In 1994, the International Astronomical Union (IAU) codified a system meant to eliminate confusion caused by duplicate names. Let's look at Comet Thatcher's official designation, C/1861 G1 (Thatcher), to explain the rules.

The most common prefixes for comet designations are C and P. A preceding P means you're looking at a periodic comet (any comet with an orbital period of 200 years or less). The number attached to the P reveals the order of discovery. For instance, Comet 1P/1882 Q1 (Halley) is so-called because it was the first comet determined to be periodic. These numbers are assigned after a comet's second perihelion passage, which (ideally) confirms the object's periodicity.

A preceding C is used for non-periodic comets, or comets with periods longer than 200 years. Comet Thatcher rounds the Sun every 415 years, so its designation begins with a C. If you see an X, you'll know that the comet's orbit couldn't be calculated. If you see an /, you'll know you've got an interstellar object (the IAU added I to the system last year after the discovery of 11/2017 U1 ('Oumuamua) In October).

Thatcher's 1861 is straightforward --that's the year of discovery. The G tells us that the comet was discovered in the first half of April (January = A+B; February = C+D; March = E+F; April = G+H; and so on). The 1 lets us know that this was the first comet discovered in the G period. And, of course, we wrap up the designation with the last name of the discoverer or the object's coziest call sign.

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Title Annotation:APRIL 2018 OBSERVING: Celestial Calendar; naming Comet Thatcher C/1861 G1
Author:Johnson-Roehr, S.N.
Publication:Sky & Telescope
Article Type:Brief article
Geographic Code:1USA
Date:Mar 18, 2018
Words:300
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