SkyEye

The Calendar

Beginning this month, the Moon occults the open star cluster Pleiades every time it passes by until 2029. The lunar occultations of Antares continue and Mars also falls victim to the Moon.

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 → Sextans → Leo

Mercury is at inferior conjunction on 6 September, after which it commences the best morning apparition of the year for early risers in northern temperate latitudes. The planet brightens throughout the month. It returns to direct motion on 15 September and reaches greatest elongation west (17.9°) one week later.

Venus Cancer → Leo

Venus quickly claims its place as the pre-eminent planetary body in the morning sky, shining at magnitude −4.5 as it appears higher in the east at every dawn. The morning star returns to direct motion early in the month and appears as a bright crescent in a telescope as it moves away from inferior conjunction.

Earth and Moon

Earth reaches its second equinox on 23 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.

The Moon indulges in a number of lunar occultations, with Neptune disappearing behind the Moon's disk on both the first of the month and on 28 September, the open star cluster known as the Pleiades vanishing on 5 September (this series of occultations will continue into 2029), Mars being blotted out on 16 September, and first-magnitude star Antares falling victim on 21 September. The Moon also passes near the first-magnitude stars Pollux (10 September) and Spica (17 September). The Full Moon on the penultimate day of the month is popularly known as the 'Harvest Moon' in the northern hemisphere, the Full Moon nearest to the autumnal equinox.

Mars Virgo

Mars is low in the west at sunset and lost in the glow of twilight for those in northern temperate latitudes, but observers in the southern hemisphere still have reasonably dark skies in which to spot the magnitude +1.8 planet. Mars pays a visit to fourth-magnitude Zaniah (η Virginis) on the fourth day of the month, third-magnitude Porrima (γ Virginis) on 12 September and fourth-magnitude multiple star θ Virginis on ten days later. Zaniah is a bluish spectroscopic binary; it should provide a nice colour contrast to Mars which is only 0.8° distant on 4 September. A lunar occultation involving the day-old crescent Moon takes place around sunset on 16 September and is visible from parts of northern South America.

Jupiter Aries

On the first day of the month, Jupiter reaches it most northly declination for the year. Three days later, the waning gibbous Moon is found 3.3° north of the bright planet. Shortly afterwards, Jupiter reaches its stationary point and goes from direct to retrograde motion. The magnitude −2.7 planet rises mid-evening and is visible until sunrise.

Saturn Aquarius

Saturn was at opposition late last month and is visible almost all night, setting during dawn skies. It is a bright magnitude +0.4 as the rings continue to open slightly, and is found 2.7° north of the waxing gibbous Moon on 27 September.

Uranus Aries

Sixth-magnitude Uranus rises mid-evening and is visible until sunrise. The waning gibbous Moon passes 2.8° south of the planet on 5 September but the brightness of our satellite will drown out the faint planet.

Neptune Pisces

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. Opposition takes place on 19 September; the planet is at its brightest (magnitude +7.8) and largest (apparent diameter 2.53 arc-seconds). The nearly full Moon occults Neptune twice this month, on the first and again on 28 September but both events are visible only from the Antarctic. However, planet watchers have dark skies for Neptune's fly-by of the fifth-magnitude star 20 Piscium. This star is a K-type giant and should provide a nice colour contrast with the blue planet.