How to find Plato, 34 Capricorni, Eratosthenes, 13 Cygni, Clavius and Eta Aquilae the weekend of September 13 - 15.
Friday, September 13 - Do you think that today is jinxed? Then your luck is about to change as we walk upon the Moon this evening and take a look at sunrise over one of the most often studied and mysterious of all craters - Plato.
Located on the northern edge of Mare Imbrium and spanning 95 kilometers in diameter, Class IV Plato is simply a feature that all lunar observers check because of the many reports of unusual happenings. Over the years, mists, flashes of light, areas of brightness and darkness, and the appearance of small craters have become a part of Plato's lore.
On October 9, 1945 an observer sketched and reported "a minute, but brilliant flash of light" inside the western rim. Lunar Orbiter 4 photos later showed where a new impact may have occurred. While Plato's interior craterlets average between less than one and up to slightly more than two kilometers in diameter, many times they can be observed - and sometimes they cannot be seen at all under almost identical lighting conditions. No matter how many times you observe this crater, it is ever changing and very worthy of your attention!
Although tonight's bright skies will make our next target a little difficult to find visually, look around four fingerwidths southwest of Delta Capricorni (RA 21 26 40 Dec -22 24 40) for Zeta? Also known as 34 Capricorni, Zeta is a unique binary system. Located about 398 light-years from Earth, the primary star is a yellow supergiant with some very unusual properties - it's the warmest, most luminous barium star known. But that's not all, because the B component is a white dwarf almost identical in size to our own Sun!
Saturday, September 14 - Tonight would be a great opportunity to take another look at crater Eratosthenes. Just slightly north of lunar center, this easily spotted feature dangles at the end of the Apennine Mountain range like a yo-yo caught on a string. Its rugged walls and central peaks make for excellent viewing. If you look closely at the mountains northeast of Eratosthenes, you will see the high peak of Mons Wolff. Named for the Dutch philosopher and mathematician, this outstanding feature reaches 35 kilometers in height. To the southwest of Eratosthenes you may also spot the ruined remains of crater Stadius. Very little is left of its walls and the floor is dotted with small strikes. Near the twin pair of punctuations to its south lie the remains of Surveyor 2!
Now let's journey to a very pretty star field as we head toward the western wing tip in Cygnus to have a look at Theta - also known as 13 Cygni. It is a beautiful main sequence star that is also considered by modern catalogs to be a double. For large telescopes, look for a faint (13th magnitude) companion to the west? But it's also a wonderful optical triple!
Also in the field with Theta to the southeast is the Mira-type variable R Cygni, which ranges in magnitude from around 7 to 14 in slightly less than 430 days. This pulsating red star has a really quite interesting history that can be found at AAVSO, and is circumpolar for far northern observers. Check it out!
Sunday, September 15 - Tonight on the Moon, let's take an in-depth look at one of the most impressive of the southern lunar features - Clavius.
Although you cannot help but be drawn visually to this crater, let's start at the southern limb near the terminator and work our way up. Your first sighting will be the large and shallow dual rings of Casatus with its central crater and Klaproth adjoining it. Further north is Blancanus with its series of very small interior craters, but wait until you see Clavius.
Caught on the southeast wall is Rutherford with its central peak and crater Porter on the northeast wall.
Look between them for the deep depression labeled D. West of D you will also see three outstanding impacts: C, N and J; while CB resides between D and Porter. The southern and southwest walls are also home to many impacts, and look carefully at the floor for many, many more!
It has been often used as a test of a telescope's resolving power to see just how many more craters you can find inside tremendous old Clavius. Power up and enjoy!
And if you'd like to visit an object that only requires eyes, then look no further than Eta Aquilae one fist-width due south of Altair...
Discovered by Edward Pigot in 1784, this Cepheid-class variable has a precision rate of change of over a magnitude in a period of 7.17644 days. During this time it will reach of maximum of magnitude 3.7 and decline slowly over 5 days to a minimum of 4.5... Yet it only takes two days to brighten again! This period of expansion and contraction makes Eta very unique. To help gauge these changes, compare Eta to Beta on Altair's same southeast side. When Eta is at maximum, they will be about equal in brightness.
Until next week? Ask for the Moon, but keep on reaching for the stars!
Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She has received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.