The Night Sky This Month: October 2015

Mons Piton
Credit and Copyright: Lunar Observatory Vaz Tolentino

Although not as spectacular as the Blood Moon eclipse at the end of September, on the 4th of October lunar observers will have the opportunity to view Mons Piton, a stand-alone mountain in the eastern reaches of the Mare Imbrium, just south east of the crater Plato, and almost due west of the crater Cassini. Mons Piton is 25 km in diameter, and its height, which was calculated by using the length of its shadow, is 2,300 meters. By lunar standards, Cassini is a rather average lava-filled crater, with a diameter of only 57 km. Nevertheless, Cassini is very old, and it bears the scars of a great many impacts, two of which are significant – Cassini A, and the smaller Cassini B.

Lunar Phases

New Moon: October 12th
First Quarter: October 20th
Full Moon: October 27th
Last Quarter: October 4th

Highlights of October

Orionid Meteor Shower

Named after their apparent point of origin in the constellation Orion, the Orionid Meteor Shower is expected to peak before dawn on the 22nd of the month. Orionids are typically fast moving, and maximum meteor counts of 10-20 per hour is common. Fortunately, the shower, which is associated with Halley’s Comet, occurs just after the First Quarter moon this time round, which means that observing conditions will not be negatively impacted by moonlight.

Zodiacal Light

Look for the cone of light in the east from about two hours before sunrise from the 11th, to the 25th of the month; however, a dark site without moonlight is required to see the reflection of light from dust particles that have been around since the formation of the solar system. Also known as the “false dawn”, the supply of dust is maintained by the collisions between asteroids, and particles shed by passing comets. Bear in mind that from around 80 minutes before dawn, the rising Sun extinguishes the phenomenon as the sky brightens.


Summer TriangleJust like last month, October’s night sky will be dominated by by the Summer Triangle, consisting of three bright stars, namely Vega in Lyra, Deneb in Cygnus, and Altair in Aquila. To the east of Cygnus can also be found the the Great Square of Pegasus, adjoining the constellation of Andromeda which contains the Andromeda Nebula (M31), while to the north lies Cassiopeia and Perseus.

The Planets


Mercury becomes a pre-dawn object around the 11th of the month, which is when it will be visible at magnitude +0.3 only 80 above the horizon about 40 minutes or so before dawn. At this point, Mercury will be about 200 or so below Jupiter, and close to a narrow crescent moon, and you may well require binoculars to find it. On the 16th, Mercury reaches its greatest western elongation, but it will soon after begin falling back toward the Sun, and even though it brightens to magnitude -1, it will become impossible to spot in the glare of the Sun toward the end of the month.


At the beginning of the month, Venus will dominate the pre-dawn sky at magnitude -4.7 around 300 above the horizon. Venus will reach its greatest western elongation on the 26th of the month, at which point its angular separation from the Sun will be 460. Although Venus rises roughly four hours before the Sun, it may be a few days before its disc will appear half-illuminated. However, its apparent diameter will decrease from 33 to 32 arc seconds during the month, but since its phase will increase from 35% to 53%, it will remain luminous, with only a slight decrease in magnitude to -4.5 toward the end of the month.


Also a predawn object during October, Mars will start the month at magnitude +1.8, and about 230 above the eastern horizon- close to the mid-point between Venus above it, and Jupiter below it. On both the 17th, and 18th of the month, Mars will shine at magnitude +1.7 less than 0.50 from Jupiter, which will be at magnitude +1.8, and whose disc will appear to be almost exactly 8 times bigger than that of Mars. During October Mars’s apparent diameter is still no more than 4 arc seconds wide, which means that no surface details will be visible.


The beginning of October sees Saturn at magnitude +0.6 in the eastern parts if Libra, around 70 above the south-western horizon roughly 45 minutes after sunset, but it will become progressively more difficult to observe as the month wears on. On the 16th, Saturn will pass into Scorpius, and by the 26th, it will be less than a single degree above Beta Scorpii. By the end of October, Saturn will be too low on the horizon to observe its 15-arc second disc and rings through the atmospheric haze, which will then be inclined to our line of sight by about 240. There is nothing for it but to wait several months for Saturn to appear above the horizon again.


At magnitude -1.7, which will increase to -1.8 as October progresses, Jupiter is one of the four naked-eye planets that are visible during the month, along with Mars, Venus and Mercury. Starting the month at an elevation of about 120 above the horizon in the east, it rises progressively earlier during the month, until it virtually collides with Mars on the morning of the 17th as it passes under the body of Leo. During October, as the Earth moves toward Jupiter, the giant planet’s disc will grow from 31.4 to 33 arc seconds which means that in good seeing conditions, early risers should have no trouble at all in observing the spectacular equatorial bands in the planet’s upper atmosphere. Also look for the four Galilean moons as they race each other around the planet.


On the 11th of the month Uranus comes into opposition, and at magnitude +5.9, it will make an easy binocular target in the southern reaches of Pisces just eastward of the Circlet asterism, and to the east-southeast of Epsilon-,and Delta Piscium- both of which will be at magnitude 4.0. Uranus will rise to a maximum 450 above the southern horizon, and given a 4-inch telescope, it should be possible to observe its 3.6-arc second disc. However, with an 8-inch instrument it might just be possible to observe at least some detail in the planet’s upper clouds, provided a green filter is used, and seeing conditions are good. With larger instruments, it might be possible to observe Uranus’s satellites Arial (+14.4), Umbrial (+15), Titania (+13.9) and Oberon (+14.1) as well.