The Dimmest Constellation

You are probably familiar with at least the names of the twelve constellations of the zodiac:


But are you familiar with the twelve constellations that have no stars brighter than 4th magnitude?

Coma Berenices
Corona Australis

All but two of these dim constellations are, at least in part, visible from southern Arizona; Chamaeleon and Mensa require a trip south to see.

The southern constellation Mensa, the Table Mountain (declination -70° to -85°) is a ghost of a constellation, exhibiting no star brighter than magnitude 5.1. That’s 17 times fainter than Polaris! In fact, that’s fainter than all the stars of the Little Dipper asterism! Mensa does have one claim to fame, however. The Large Magellanic Cloud, satellite galaxy of our Milky Way galaxy, straddles most of the border that Mensa shares with Dorado, the Swordfish.

Mensa is far and away the dimmest constellation. But Mensa is a small constellation, bested in size by 74 of the 88 constellations. So perhaps it is not too surprising that a small constellation is less likely to harbor a bright star. Another measure of faint, perhaps, is to determine which of these twelve constellations with no star brighter than 4th magnitude is largest. That might be more remarkable, because one is less likely to find no bright stars in a large area of sky than in a small area of sky. By this measure, Camelopardalis, the Giraffe, wins without a doubt. Camelopardalis is the 18th largest constellation, and yet contains no star brighter than magnitude 4.0. It is that empty region you might have not noticed midway between Capella and Polaris, best viewed at evening twilight’s end during the month of February each year.

February is Short, the Moon Makes Haste…

Each night for the next several nights, the Moon sets much later than it did the previous night. This happens for two reasons.

First, this week the plane of the Moon’s orbit is nearly perpendicular to our horizon, so much of the Moon’s orbital motion eastward relative to the background stars (if we could see them) during the day takes it directly away from the western horizon, thus slowing as much as possible its inexorable march towards the west caused by the Earth’s rotation.

Second, this week the Moon is moving north in declination, and this, too, increases the amount of time the Moon stays above the horizon. The closer to the north celestial pole an object is, the longer it stays above our horizon, the further north along the western horizon it sets, and the later it sets. The Moon’s motion during the day northward relative to the celestial equator causes the Moon to set further north than it would have otherwise. The combination of these two factors makes moonset much later each night, as shown in the table below.

But, why doesn’t moonrise also occur much later each morning? As you can see by inspecting the table above, the Moon rises only a little later each day, in marked contrast to the leaps and bounds moonset is later each night. The factors are the same, but the effect is different. Because the Moon is moving north and is thus rising further north every morning, it rises earlier than it would have otherwise. Although the Moon is rising later each day due to its eastward orbital motion, moonrise is only a little later due to the countereffect of an earlier rise time stemming from the Moon’s more northerly declination.

It is no wonder humans have always been fascinated by the Moon’s complex motion. Throughout history, a number of mathematicians have taken up the challenge of trying to understand and predict the Moon’s motion, leading to several important advancements in mathematics.