MoOnday 2016 – 15 February 2016

Date:      MoOnday 15 February 2016
Time:      17:00 – 23:00 (all times are GMT)
Place:     The Hub Cafe, Regent’s Park, NW1 4RU with the Baker Street Irregular Astronomers

The Sun sets today at 17:10.

The Moon has been in the sky all afternoon, in the constellation of Taurus. It is close to the bright red star Aldebaran and the famous star cluster of Pleiades. Moonrise was at 10:54 and it will cross the meridian (ie will be due south) at 18:24 when it will be 374,581 km from Earth and 55.1% illuminated. The Moon was precisely at First Quarter (50.0% illuminated) at 07:46, before it rose this morning. Since that time it has started to wax. The Moon will set early tomorrow morning at 00:53.

What does the Moon look like when you stare at it?

First Quarter Moon by Guy Wells

First Quarter Moon by Guy Wells

You will see the Moon as in this photograph. Some areas are light and same patches are darker. Why are the dark patches called ‘seas’? The lighter areas are highlands.

Note the division between the illuminated right side of the Moon and the left side in shadow. The division between the two is called the terminator.

At First Quarter the terminator is straight vertically. Note how the shape of the terminator will change over the next few days.

What do you see when you look through a pair of binoculars or a telescope?

First Quarter Moon by Guy Wells

Looking through a pair of binoculars you see something like this image. If you are looking through a telescope, owing to the instrument’s optics, the image will be either be inverted or flipped left to right. Either way you will see greater detail in the image.

Can you see six maria?

Frigoris (Cold) at the top (north),
Serenitatis (Serenity) below.
Below them is Tranquillitatis (Tranquillity) where Man first walked on the Moon in July 1969.
Nearer the edge (limb) are Fecunditatis (Fertility) below the easily seen oval Crisium (Crises).
The most southern ‘sea’ visible is Nectaris (Nectar).

Notice how craters visible are of different sizes and some are superimposed on others. Shadows cast by the edges of craters reveal their depths, see how some are deep, others shallow and many have a walled rim with a central mountain peak. Some of the ancient craters have been flooded by lava. Studying craters help astronomers understand the geology and age of the Moon.

10 Lunar Features To Observe


Craters Aristoteles and Eudoxus By Michael Karrer

Craters Aristoteles and Eudoxus By Michael Karrer (CC NC2.0)

Just below the most northern ‘sea’ Mare Frigoris are two easily seen craters.

ARISTOTELES is a very prominent, young, large 87 km circular crater formed 3.2 – 1.1 billion years ago. It has pronounced terraced inner crater walls, and the crater floor lies 3.3 km below the crater rim. Remnants of central mountains are detectable.

Forms a remarkable couple with EUDOXUS, another prominent crater with terraced crater walls. It has a diameter of 67 km and a depth of 3.4 km.

Minimum Objective needed to observe: 50mm.


Montes Haemus (NASA/LROC)

Montes Haemus (NASA/LROC)

A young impact crater, 27 km diameter, located on the southern shore of Mare Serenitatis near the eastern end of the Montes Haemus mountain range. The wall of Menelaus is slightly irregular in outline, with a high, sharp rim and terraced inner walls. Formed by low angle impact 1.2 billion years ago

Minimum instrument: 50mm.


Theophilus, Cyrillus & Catharina by Michael Karrer (CC NC2)


A 101km crater formed 3.2-1.1 billion years ago.

Circular formation with tormented and steep slopes overhanging Sinus Asperitatis to the north from 1200 km and supporting CYRILLUS to the south-west and the crater MADLER to the east. Very high walls with terraces overlapped by THEOPHILUS B to the north-west. Flat floor. Imposing central mountain 1400m high with four summits. Line of crests hills and craterlets.

Minimum instrument: 10x binoculars.


A 98 km crater formed 3.9-3.8 billion years ago. Has a damaged circular formation. Tormented and steep slopes supporting THEOPHILUS to the north-east and the crater IBN RU’SHD to the north-west. Pretty high walls higher to the east than to the west crushed by THEOPHILUS to the north-east. Tormented and extensive floor with three central mountains and a groove to the south.

Minimum instrument: 10x binoculars.


Formed during the same era as CYRILLUS, this crater is 1000 km wide having a very damaged circular form. Pretty steep slopes and tormented to the north-east and supporting minor craters. High walls to the south-east crushed by the large ghost semi-crater CATHARINA P to the north, by CATHARINA B and G to the north-east and by CATHARINA S to the south. Flat floor to the east and tormented to the west.

Minimum Instrument: 10x binoculars.


Ptolemaeus, Alphonsus, Arzachel by David Evans (CC A2)

Ptolemaeus, Alphonsus, Arzachel by David Evans (CC A2)

These ancient, large impact craters form a beautiful triplet are on the north eastern shores of Mare Nubium, positioned near to the terminator. The large walled plain Ptolemaeus (or Ptolemy, to give it its more familiar English name) has a distinctly hexagonal appearance.

Ptolemaeus has a large flat floor with no central peak , and the edge is almost nonexistent in some places. The middle crater, Alphonsus, has also a flat floor, but shows a central peak and its walls are more robust. The southern one, Arzachel , is a fine example of a complex crater, with terraced walls, and a distinct central peak.

In terms of age of craters, Ptolemaeus is the oldest, Alphonsus is the middle one and the youngest is Arzachel. This group is a favourite with lunar sketchers and photographers.

Minimum instrument: 10x binoculars.


Maurolycus by David Campbell

114 km in size and formed 3.9 – 3.8 billion years ago. A circular formation forming an interesting couple with crater BAROCIUS. It has very steep slopes especially to the east supporting smaller craters to the north, BAROCIUS to the south and presenting a double wall to the south-east. It has a flat floor with numerous craters and an off-centred mountain

Minimum instrument: 10x binoculars.