February 1999

"Once in a black moon ?"

by Steve Stefanik

"Once in a blue moon" we have two full moons in one month like we did last month. However, because the second full moon occurred on January 31 and because the lunar cycle is only 29 1/3 days, that means that this month there will not be a full moon because February only has 28 days! The question then is, how often does this occur?

This of course means that the next full moon will be on the second day of March which means that there will be yet another "blue moon" on the 31st of March. Actually this occurs not that infrequently. This happens 10 times in the next 20 years. Once in 2004, 2007, 2009, 2012, 2015, 2020 and twice in 2001 and 2018. So if a month in which there is no full moon can only occur in a February (that only has 28 days and not 29 as happens in a "leap year") when the preceeding January has a "blue moon", the next time a month will occur without a full moon will be in February of 2018, once in the next 19 years or "once in a black moon"! (You heard it here first.)

The main astronomical event of the month doesn’t involve the moon however. It involves the conjunction of the planets Venus and Jupiter. As the month of February begins the planet Jupiter is still fairly high in the southwestern sky at nightfall. Its the -2.1 magnitude bright object below the "Great Square" of the constellation Pegasus. Venus on the other hand is the brilliant -3.9 magnitude "star-like" object low on the western horizon as the sun goes down. The pair of planets are separated by 22 degrees at the start of the month but because Venus is hurtling toward us in its shorter faster orbit it is closing the apparent gap with Jupiter by one degree per day. By mid-month they will be only 7 degrees apart. By the last week of February they will close to within one degree of one another. On the evening of the 23rd they make their closest approach, a mere 11 arc minutes from each other. The twosome should fit into the same field of view using binoculars or a small telescope. This will make a great photographic opportunity for those of you with the capability of mounting a camera on a tripod. Use a "slow speed" film and adjust the aperature on your lens to wide open and bracket your shutter so that exposures start with 1/30 of a second and go down to 1 second. By the end of the month the pair will once again be separated by 5 degrees but Jupiter will be lower than Venus on its way to setting next month.

The planet Mercury joins the fray but is only 4 degrees above the horizon. Jupiter and Venus should help you locate this dimmer -1.0 magnitude elusive planet. Look half-way between them and the western horizon.

Saturn is 27 degrees above and to the left of Jupiter and Venus. Look for the paler +0.3 magnitude planet about 3 degrees north of the waxing crescent moon on the evening of the 20th if you have difficulty locating it before then.