SkyEye

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 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.

Sun Leo → Virgo

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

Mercury Leo → Virgo

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 Gemini → Cancer

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.