September 2018

Welcome to SkyEye, your guide to this month's celestial events.

The Calendar

Saturn's rings are at their most open this year. This coincides with quadrature, making the ringed planet a particularly tempting telescopic target this month. Can you see the shadow of the planet cast upon the rings?

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Date Body Event
2 Mercury perihelion
Venus 1.4° south of Spica
3 Moon occultation of Aldebaran: visible from northern Canada and Greenland.
Moon last quarter
5 Venus aphelion
Mercury 1.0° north of Regulus
6 Saturn stationary point: retrograde → direct
Moon ascending node
7 Moon 1.1° south of the open star cluster M44 (known as Praesepe or the Beehive Cluster)
Neptune opposition
8 Moon perigee
Moon, Mercury occultation of Mercury - not visible
9 Moon new
16 Mars perihelion
Moon last quarter
19 Saturn maximum ring opening (26.6°)
20 Moon apogee
Moon descending node
21 Mercury superior conjunction
23 Earth equinox
25 Moon full
Saturn east quadrature
27 136472 Makemake conjunction

The Solar System

The word planet is derived from the Greek word for 'wanderer'. Unlike the background stars, planets seem to move around the sky, keeping mostly to a narrow track called the ecliptic, the path of the Sun across the stars. Dwarf planets and small solar-system bodies, including comets, are not so constrained, often moving far above or below the ecliptic.

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Sun LeoVirgo

The solar north pole is most inclined toward the Earth early this month.

Mercury LeoVirgo

Observers in the southern hemisphere will have little luck seeing Mercury this month but the planet will be on show just before sunrise during the first few days of September for those in the north. Look for it very near the bright star Regulus on 5 September. At superior conjunction on 21 September, the closest planet to the Sun will reappear next month in the west at sunset.

Venus VirgoLibra

The evening star heads back toward the western horizon but is still quite high as seen from southern latitudes. On the second day of the month, Venus appears less than 2° away from the first-magnitude star Spica. Later in the month, on 25 September, Venus reaches a magnitude of -4.6.

Earth and Moon

On 23 September, Earth reaches equinox. The word equinox means 'equal night' so that on this day, the (centre of the) Sun spends an equal amount of time above and below the horizon everywhere on the planet. In the northern hemisphere, the nearest Full Moon to the autumnal equinox is called the 'Harvest Moon'. This year, the 'Harvest Moon' occurs on 25 September.

The path of the Moon across the sky occasionally takes it in front of a first-magnitude star. On 3 September, just such an occultation occurs when the Moon obscures Aldebaran in the constellation of Taurus. This is the last in a series of 49 occultations which began in January 2015. The next Aldebaran series won't begin until August 2033. The next first-magnitude star to be occulted by the Moon will be Antares in 2023.

Mars Capricornus

Bright Mars continues to put on a good show for observers in equatorial and southern latitudes.

Jupiter Libra

Jupiter is high in the west at sunset when observed from the southern hemisphere but is getting increasingly difficult to view from the north.

Saturn Sagittarius

This is an excellent month in which to observe Saturn telescopically. The rings are at their most open as seen from Earth and Saturn reaches east quadrature on 25 September. It is at this time that the interplay of planet, moon and ring shadows are the most complex and interesting. Saturn reaches a stationary point on 6 September and resumes direct motion across the sky.

Uranus Aries

Uranus rises in mid-evening and is getting ever brighter as it approaches opposition next month.

Neptune Aquarius

A small telescope is necessary to view the most distant planet in the solar system. Opposition is on the seventh day of the month so this blue ice giant is aloft most of the night.

The Celestial Sphere

Constellations are patterns of stars in the sky. The International Astronomical Union recognises 88 different constellations. The brightest stars as seen from the Earth are easy to spot but do you know their proper names? With a set of binoculars you can look for fainter objects such as nebulae and galaxies and star clusters or some of the closest stars to the Sun.

Descriptions of the sky for observers in both the northern and southern hemispheres are available for the following times this month. Subtract one hour from your local time if summer (daylight savings) time is in effect.

Local Time Mid-month Northern Hemisphere Equator Southern Hemisphere
1730 hours (1830 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S
1930 hours (2030 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S
2130 hours (2230 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S
2330 hours (0030 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S
0130 hours (0230 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S
0330 hours (0430 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S
0530 hours (0630 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S