Wednesday, July 31, 2019

Can You See The "Black Moon" Today? Nope - Just Know It's Occurred

Ok, first the good news: there will be a black Moon occurring all across  North America today,  July 31st -- the first one since 2016.  The bad news? You won't be able to actually see it because all it means is there is a second New Moon.  This is not visible given it is in the position shown in the diagram below:
So, while a 'blue Moon' derives from its rarity - the 2nd full Moon in a month, the  black moon is basically the second new moon of the month.  This is also something that rarely occurs but we use the word 'black' because, well, the Moon's non-sunlit side is facing Earth,  so 'black' to us. Get it?.
Another aspect to why this is possible lies in the difference between the Moon sidereal period (27.3 days), i.e. referenced to the distant stars, and its longer synodic period (29.5 days) referenced to its position relative to the Sun, or phase.  This difference is captured in the diagram below:
Over the period of the sidereal revolution, then,  Earth and Moon together revolve roughly 1/13 way around the Sun.  This is an angular distance of about 360 deg/ 13 =  27.6 degrees. In effect, the Sun then appears to move 27.6 degrees east on the celestial sphere over the Moon's sidereal revolution.  In other words, the Moon would not have completed a whole revolution about the Sun over the sidereal period.  Consequently it would not have completed a full cycle of phases (e.g. New to New, full to full).  A fractional phase increment is always left over.

 The  slight deviation in position, between synodic and sidereal, means certain observational anomalies can occur over time, and given the lengths of months can vary, i.e. 28 or 29 days for February, and 30 or 31 days for other months. Combine the two effects and you can get two full Moons some months, and also two New Moons, i.e. because the months in the latter cases (31 d) are slightly longer than the Moon's synodic period.
 So about every 32 months we can get two full moons or two new moons. The second full moon in a month is called a blue Moon, and the second new moon is called a black Moon. 

 Oh, by the way, "black Moon" can also refer to no New Moons in a given month!

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