Monday, 2008 September 22, 23:45 — astronomy

Martian months

Most proposed calendars for Mars have 24 months, and various systems have been offered to name them. Here’s one more: use the names of the 24 brightest stars, in order of longitude right ascension (relative to the rotation axis of Mars), so that each star is conspicuous at night in the month named for it.

  • Vega (α Lyrae), Altair (α Aquilae), Fomalhaut (α Piscis Austrini), Deneb (α Cygni), Achernar (α Eridani), Aldebaran (α Tauri), Rigel (β Orionis), Capella (α Aurigae), Betelgeuse (α Orionis), Canopus (α Carinae), Sirius (α Canis Majoris), Adhara (ε Canis Majoris), Castor (α Geminorum), Pollux (β Geminorum), Procyon (α Canis Minoris), Regulus (α Leonis), Arcturus (α Boötis), Spica (α Virginis), Mimosa (β Crucis), Acrux (α Crucis), Hadar (β Centauri), Rigil Kentaurus (α Centauri), Antares (α Scorpii), Shaula (λ Scorpii)

But some of these stars are at high latitudes declinations; a star near the pole does not move much and so makes an inferior signpost, as well as being out of view to large regions. So I weighted brightness according to the cosine of declination, and got this list:

  • Vega, Altair, Alnair (α Gruis), Fomalhaut, Achernar, Aldebaran, Rigel, Bellatrix (γ Orionis), Capella, Elnath (β Tauri), Betelgeuse, Canopus, Sirius, Castor, Pollux, Procyon, Regulus, Arcturus, Spica, Acrux, Hadar, Rigil Kentaurus, Antares, Shaula

Unfortunately for this purpose, bright stars are not evenly distributed: the gap between Achernar and Aldebaran is almost six Martian months, while that between Arcturus and Spica is one day. So, starting with a list of 300 bright stars, I looked for the smallest RA gap and removed the dimmer star (as weighted), repeating until 24 remained:

  • Vega, Altair, Alnair, Fomalhaut, Achernar, Markab (α Pegasi), Diphda (β Ceti), Alpheratz (α Andromedae), Hamal (α Arietis), Mirfak (α Persei), Capella, Sirius, Procyon, Dubhe (α Ursae Majoris), Regulus, Alioth (ε Ursae Majoris), Alkaid (η Ursae Majoris), Porrima (γ Virginis), Arcturus, Muhlifain (γ Centauri), Acrux, Rigil Kentaurus, Antares, Shaula

The widest gap is 46.6 days, the narrowest 15.4 (a bit over half a month).

I thought of this years ago, but only now got around to writing the code to generate the list. (I have no obvious way of checking its accuracy.)

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