September 2020

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

The Calendar

Venus continues its domination of the dawn skies as the morning star but is appearing a little closer to the horizon every day. Like last month, Mars is occulted by the Moon. Neptune reaches opposition and is at its brightest as seen from Earth.

The phases of the Moon in September 2020

Date Body Event
2 Mars solstice
Moon full
6 Moon, Mars occultation of Mars — visible from central South America, central Atlantic and northwestern Africa
Moon apogee
8 Mercury descending node
9 Mars stationary point: direct → retrograde
10 Moon last quarter
11 Neptune opposition
13 Jupiter stationary point: retrograde → direct
14 Moon 1.8° north of the open star cluster M44 (known as Praesepe or the Beehive Cluster)
17 Moon new
18 Moon perigee
19 Mercury aphelion
22 Mercury 0.3° north of Spica
Earth equinox
23 Moon descending node
24 Moon first quarter
25 Moon, Jupiter 1.6° apart
26 Venus ascending node
29 Saturn stationary point: retrograde → direct
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.

The position of the Sun and planets at mid-month

Sun LeoVirgo

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

Mercury LeoVirgo

Mercury reaches another aphelion this year on 19 September. Three days after that, the zero-magnitude planet is found 0.3° north of Spica, the brightest star in the constellation of Virgo. An evening sky object, Mercury is just about unobservable from northern temperate latitudes during this apparition. On the other hand, this is an excellent opportunity for planet watchers in the southern hemisphere to espy this usually difficult object as it soars high above the western horizon.

Venus GeminiCancer

The morning star is descending back toward the eastern horizon, appearing a little lower before sunrise every morning. This apparition favours the northern hemisphere where Venus has attained 40° or more in altitude. When viewed through a telescope, Venus appears in its waxing gibbous phase but its apparent angular diameter is diminishing as the planet leaves Earth behind. The bright planet dims slightly this month, from magnitude −4.2 to −4.1.

Earth and Moon

On 6 September, Mars is occulted by the waning gibbous Moon. Eight days later, the waning crescent Moon passes within 2° of Praesepe (or the Beehive Cluster). Earth reaches its second equinox on 22 September. 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.

Mars Pisces

The second day of the month marks the arrival of winter in the northern hemisphere of Mars and summer in the south. Four days later, the waning gibbous Moon occults the red planet. The event begins at around 02:30 UT and is visible from central South America, the central regions of the Atlantic Ocean and northwestern Africa. Mars reaches a stationary point on 9 September and begins retrograde motion ahead of its opposition next month. Look for the rapidly brightening planet from the early evening onwards.

Jupiter Sagittarius

Jupiter resumes direct motion on 13 September and the waxing gibbous Moon pays a visit 12 days later. The bright gas giant fades slightly this month, from magnitude −2.6 to −2.4, and is visible as the sky darkens in the evening. The best views are from the southern hemisphere.

Saturn Sagittarius

Like Jupiter, the ringed planet also has a close encounter with the waxing gibbous Moon with our satellite passing just over 2° south of Saturn on 25 September. Four days later Saturn completes its retrograde motion for the year and returns to a direct path amongst the background stars. The rings have been opening up since their minimum tilt in May and reach a local maximum of 22.8° on the last day of the month. The rings then begin to close again. Look for Saturn in the evening sky, not far from the somewhat brighter Jupiter.

Uranus Aries

Uranus rises a little earlier every evening but the best views still occur after midnight on moonless nights.

Neptune Aquarius

A small telescope is necessary to view the most distant planet in the solar system and this is the best time this year to observe Neptune. The blue ice giant reaches opposition on 11 September. When viewed through a telescope, Neptune is only 2.5 arc-seconds in apparent diameter and shines at magnitude +7.8. It rises at sunset and is up all 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