Hydra shows her head this month between Canis Minor and Leo, and Praesepe is the jewel of Cancer
Sun: The vernal equinox is the 20th, at 16.57 GMT, or 09.57 MDT
Moon: New, 1st and 301th; First Quarter, 8th; Full, 16th; Last Quarter 24th. The Moon is at perigee on the 27th, apogee on the 11th. The second full moon this month is a blue moon.
Mercury: is visible in the morning all month.. Greatest elongation /west of 28° on the 14th. This is very good for seeing Mercury, but it is in tises several hours after midnighthe early morning.
Venus: is the Morning Star, Phosphorus, until Autumn.
Mars: in the southeast at sunset. about 2 hours ahead of Saturn..
Jupiter: is high in th southwest after sunset.
Saturn: rises after midnight in the east, two hours after Mars.
March, the month of Mars, in which the vernal equinox lies, has often been the first month of the year. Before the calendar reform of 46 BCE, the solar year of 12 months had 355 days. March, May, July and October had 31 days (they still do), February had 28, and the rest had 29. Note that September, October, November and December are numbered from March, not January, though January was then considered the first month of the year. Every two years, an intercalary month of 20 or 21 days was inserted after 23 February to bring the seasons back in register. The Julian calendar brought order to this confusing chaos, with months of the present length, leap years, and the beginning of the year on 1 January. This calendar was based on Egyptian precedents, and was devised by astronomers, not Julius Caesar, who was only a supporter. The 365.25 days in the year was close enough for practical purposes to the actual length of the year. The present Gregorian calendar that omits 1 in 400 leap years is an unnecessary complication. In the southern hemisphere, Christmas occurs in midsummer, and Easter in the autumn, so any exact correspondence of festivals and seasons is ruined anyway. March was considered the first month of the year in England, for some reason I do not know, until England adopted the Gregorian calendar in 1752. This has given rise to tedious dating problems, including the relation of Newton's birth (25 December 1642, O.S.) and Galileo's death (8 January 1642, N.S.). Greece grudgingly accepted it in 1923, but the Julian calendar is still used in the Orthodox church. Gregorian dates are also designated A.D. and B.C.
March is named after Mars, mensis Martis. The word "calendar" itself comes from the Kalendae, the first day of a month in the Roman calendar, on which the priests charged with the task announced the dates of the Nones and Ides for that month, and the festival days. It is one of the handful of words to have retained the K after a general change to C. Debts were traditionally paid on the Kalends, which gave rise to the phrase "Greek Kalends" to express the fact that Greeks ignored their debts, since there were no Kalends in their calendar. A calendarium was, in fact, an account book.
We now notice 1st-magnitude Regulus, the Little King, rising in the East in the early evening. This star marks the head of Leo, forming the handle of the familiar sickle-shaped asterism. The first stars of Leo that are visible in the east in the evening are Regulus and γ, Algieba, the brightest stars in the sickle. Leo will be considered next month. We should already recognize the prominent Winter Triangle of Betelgeuse, Procyon and Sirius. To the east of this is another prominent triangle, lying in the region of Hydra and Cancer, that is very useful for locating these constellations. It is an isosceles triangle with vertices Pollux, Regulus and Procyon. In the middle of the side Procyon-Regulus, α and β Cancri are found halfway between Praesepe to the north and the head of Hydra to the south. α is to the east and β is to the west, brighter than the other stars in the area at magnitude 3.5. These represent the claws of the crab. The line α--β points almost directly at Procyon. Praesepe is in the middle of the side Pollux-Regulus.
Just above α and β is the rest of the inconspicuous zodiacal constellation of Cancer, the Crab. There are no bright stars at all in this area, but a keystone-shaped quadrilateral formed by δ, θ, η and γ frames a wonderful sight, the galactic cluster called Praesepe (three syllables), the Manger. δ and γ represent burros feeding at it. In Messier's catalog of comet-like objects, it is M44. Far from artificial lights, it can be seen with the unaided eye as a faint cloud. In ancient times, it was used as a weather predictor. When the high, thin cirrus clouds of an approaching front raised the background illumination of the sky, Praesepe disappeared, although the presence of the thin clouds was otherwise undetectable. In Denver, of course, it is permanently invisible without aid because of the sky brightness. When you turn your binoculars on it, however, its magnificence can be glimpsed. You can resolve individual stars; the brightest of these are around sixth magnitude. Galileo was the first to see the cloud resolved into stars, in 1610, when he turned his new telescope on it. Praesepe contains about 200 stars, of which 15 are brighter than magnitude 7.5. The brightest is 6.3.
Half-way between Procyon and Regulus, below Cancer, is the Head of Hydra, Caput Hydrae, a delightful ring of five stars. There are two stars at each end, and one star on the north western side marking the nostrils. The star to the east of the head, the brightest in the area at 3rd magnitude, marks the nape of the neck. This all fits nicely into the field of view of binoculars, and is instantly recognizable. A little way to the south east by way of two intermediate stars is α Hydrae, the Heart of Hydra, Cor Hydrae. This golden-yellow 2nd-magnitude star is easy to identify because it is in a large region containing no other stars nearly so bright. In Arabic, it is called Alphard, the Solitary One, for this reason. These things are the western part of a long, narrow constellation winding from 8h to 15h right ascension, the tip of the tail not far from Antares. Hydra will be with us until high summer, lying along the southern horizon (for me), her dragon's head raised in the west.
South of Hydra, and east of the bright stars of the Greater Dog's hindquarters, are the stars of Puppis, Pyxis and Antlia. Puppis extends farther south, nearly to Canopus, and 2nd-magnitude ζ Puppis can be seen about on the meridian at 9 pm on the 15th, 10° above my horizon. Two bright stars in Vela are yet further south and a little east, making an isosceles triangle with one vertex of the base at ζ. This is the region of the Milky Way, and the edge of a rich field of stars visible from the southern hemisphere. Pyxis shows three 4th-magnitude stars in a line pointing NNE, while Antlia further east contains hardly anything of note.
Don't confuse Hydra, Hydrae (Hya) with the constellation Hydrus, Hydri (Hyi) which is near the South Pole and contains no bright stars. Hydra is the largest of all the 88 constellations (Virgo is second, Ursa Major third, and Cetus fourth). While we are talking about confusions, The August constellations Sagitta (Sge) and Sagittarius (Sgr) can be mentioned. The letters in parentheses are the usual abbreviations for the genitives.
Praesepe, as well as the Pleiades and Hyades in Taurus, at which it would be good to take a final look now before they disappear in the west, are examples of galactic star clusters. They are called galactic because they generally are close to the galactic plane, and are often easy to resolve into individual stars. They are not at the tremendous distances of the globular clusters, and represent stars that formed at about the same time from the same cloud of stellar material, usually now condensed entirely into stars or blown away. Praesepe is about 525 light-years distant. The great globular cluster in Hercules, M13, is 25,000 light years away. The closest galactic cluster includes many of the stars of Ursa Major, as well as Sirius, and is not recognizable as a cluster at all, except by the common motion of its stars. The Sun is passing near or through this cluster, but is not a member of it.
The horse-taming Twins or Dioscuri, Castor and Pollux, are easily recognized as a bright pair in the eastern sky. Castor, magnitude 2.0, rises first, and then Pollux, magnitude 1.1. Castor is α, and Ptolemy reckoned its magnitude at least equal to that of Pollux, which is β. The stars are 4.5° apart, each has a third-magnitude companion (not physically associated), and each is the head of two chains of stars stretching in the direction of Orion, representing the bodies of the Dioscuri, facing each other. γ is the brightest star in this latter area, where a transverse line of stars suggests feet. To the north of Gemini is nearly nothing, the exceedingly faint constellation Lynx. It has only one bright star, α, over on the way to Leo, magnitude 3, almost at the eastern boundary of the constellation. We will find this star next month.
At 18h 26m GMT on 20 March (2007) the apparent (real) sun will cross the equator from south to north in its eastward journey along the ecliptic. The sun will rise almost due east, and set almost due west, so that the day will be 12h long. The imaginary point where the equator and ecliptic cross is the Vernal Equinox or First Point of Aries (though now actually in Pisces). Its symbol is . This means that the opposite crossing, the Autumnal Equinox, in Virgo near Denebola in Leo, will be on the meridian at midnight. The sun will next occupy the Vernal Equinox 365.2421897 days later, in March 2002. This is the tropical year, and is not the exact period of earth in its orbit because of the precession of the equinoxes, about 50" westward during the year along the ecliptic, so that it meets the equinox 0.01417 day, or about 20.5 minutes, before completing one orbit and one sidereal year. The earth's pole is describing a circle of radius 23.5° on the celestial sphere as the earth precesses because of the forces of sun and moon on the equatorial bulge. A complete cycle occupies 25,800 years. This only changes the coordinates of the stars, not their actual locations. The ecliptic remains in the same position relative to the stars. This month the ecliptic is seen at its highest (the Summer Solstice) near the feet of Gemini, then through Praesepe to Regulus, which is almost on the ecliptic. 2000 years ago, the solstice was in Cancer, for reasons we have discussed above. Precession of the equinoxes was discovered (and accurately determined) by Hipparchus in the second century BC by comparing Babylonian observations with his own.
This month offers a very good example of how to locate objects in the sky. If you look up and find Pollux, Procyon and Regulus, you can form a good idea of where to find Praesepe or the head of Hydra, and can imagine their locations even if they are not clearly visible without optical aid. Raising the binoculars, you will then probably find the one you desire in short order. The first things to do when you begin to observe are to determine north, then to identify the bright stars and the patterns they form. Now you have a framework for locating things. Since we are interested mainly in binocular views, this is a far as we usually go in these pages. To find finer objects, you then need to know the area of the sky involved, and use the patterns of the stars in the same way, but on a local scale. Computer-directed telescopes might seem an alternative, but they are very difficult to use, especially for the uninitiated, and are wasted in areas with poor observing conditions.
At 9 pm on the 15th, a belt of bright stars adorns the Milky Way from Aldebaran in the north west to α and β Centauri, twin bright stars, in the south east. The Winter Triangle is evident to the north west. Spica is due east, and Achernar is south west, both at about 20deg; elevation. Regulus is in the north east, with the sickle of Leo, while brilliant Canopus is a little south west, at an elevation of 61deg;. Directly in the zenith is the fourth-magnitude α Pyxis. Just south of the zenith is a roughly equilateral triangle formed from ζ Puppis, γ Velae and, λ Velae, and further south another from α Velae, ε Carinae and ι Carinae. With the star β Carinae, some distance to the south, the latter triangle makes a kite-shaped figure.
Return to The Night Sky
Composed by J. B. Calvert
Created 26 December 2000
Last revised 11 September 2012