Notes on the Planets in September 2009

MERCURY sets about two and a quarter hours after the Sun on September 1. 45 minutes after sunset it will 16° above the horizon and almost due west. At magnitude 0.6 it will be readily visible as the sky darkens.

The planet will get steadily lower and become less bright over the course of the following evenings. By September 12 it will set about 90 minutes after the Sun and be only 7° up 45 minutes after sunset and have dimmed to magnitude 2.0. So it will then be a difficult object in the twilight, and will be entirely lost to view within the next day or so.

Mercury is at inferior conjunction between the Earth and Sun on September 20 after which it will move into the morning sky, rising shortly before the Sun, but will be too low for observation.

VENUS, will rise before the Sun throughout September, so remaining a morning object. But at only 80 minutes earlier at the beginning of the month, and 50 minutes earlier at the end, it will be a low, but brilliant object in the dawn sky, visible some way round towards the north from east.

On the morning of September 17, the thin crescent Moon will be less than 2.5° to the upper right of Venus, but the two will be very low in a strong dawn sky glow.

MARS remains as a morning object throughout September, but does start to rise a little earlier as the month progresses. It rises at 3.10 am on September 1 at Auckland, and about 45 minutes earlier by the end of the month (but by virtue of the start of NZDT, 15 minutes later by the clock). The planet is some distance north of the equator, as a result it rises later further south in New Zealand. At Dunedin it rises about 45 minutes later than at Auckland.

During September, Mars makes its way across Gemini, ending the month just over 6° above Pollux, at magnitude 1.2 the brightest star in the constellation. Mars will be a little brighter than the star, at magnitude 0.8. The 38% lit Moon passes within 2° of Mars as seen from NZ on the morning of September 14. The distance they are apart is only 4 times the diameter of the full Moon, from the Arctic the Moon will be seen to occult the planet.

JUPITER will be easily visible all evening and on into the early morning hours during September. By mid to late evening it will be high in southern skies. It transits, so is due north and highest, soon after 11pm on September 1, and soon after 10pm (NZDT) by the 31st. During September Jupiter will move a little further away from Neptune, due to its higher retrograde (westerly) motion. By the end of the month the two planets will be over 6.5° apart, both still in Capricornus.

The nearly full Moon passes Jupiter on the night of September 2/3, with the two about 6° apart early evening and less than 4° apart about 5 am, shortly before they set. A second pass of the Moon occurs on the last evening of September when Moon and Jupiter will be 6° apart early evening. At the same time the Moon will less than 3° from Neptune.

Also on September 3 there will be a short period when none of the 4 Galilean satellites of Jupiter is visible clear of the planet. Such events are quite rare. At this event Europa and Ganymede will transit across the face of Jupiter, while Io and Callisto will be behind Jupiter or eclipsed by the planet's shadow.

The last satellite to disappear from view will be Ganymede which will begin to transit across the face of Jupiter at about 4.45pm, NZST (04:45 UT). The first to reappear will be Io from eclipse just after 6.25pm NZST (06:25 UT). Unfortunately for New Zealand viewers, most of this time span is before sunset. For much of NZ sunset time is close to 6 pm, ranging from 5:47pm at Gisborne to 6:19 pm at Invercargill. Some quick telescope viewing very soon after sunset will be needed to capture the event. The gradual reappearance of Io from eclipse should be detectable from most parts of NZ. It will occur between 6.28 and 6.30 pm close to Jupiter's eastern limb.

SATURN will be at conjunction with the Sun on September 18, so will be quite close to it all month. On September 1 the planet sets just over an hour after the Sun so it will be briefly visible very low, about 6° up half an hour after sunset, in the western sky. It will daily get more difficult to see in the twilight.

The early setting of Saturn is unfortunate for ring watchers. The Earth passes through the plane of Saturn's rings on September 4. Up to that date, and since February 1996, the southern face of the rings has been in view. After passing through Saturn's ring plane, the northern face will come into view and remain so until March 2025.

The ring plane crossed the Sun on August 10, so since that date the northern face of the rings has been lit. As a result on September 4 while the rings are likely to become invisible from the Earth, the shadow of the rings on Saturn may be just visible as a very thin dark line.

After conjunction Saturn will become a morning object rising just before the Sun but will be too low in the morning twilight to observe for the rest of the month.