Saturn and Venus

Dia 4 Looking NEE at 00h 00m GMT (UT) on August 12 showing the tracks of Saturn and Venus to 1 September

Both Saturn and Venus are readily seen in the early mooring sky. (See archive, Venus in the morning sky.)

Continuing our theme on conjunctions, Venus will be 2° south of Saturn on August 31. Dia. 4 shows the tracks of Venus and Saturn from August 12 to September 1. Note how much more rapidly Venus appears to travel than Saturn.

Appulses, conjunctions and occultations.

The transit of Venus in June this year has prompted a number of people to ask why such phenomenon occur so infrequently? Transits of Mercury and Venus have already been discussed (see archives).

When two bodies come to occupy the same celestial longitude (or Right Ascension) they are said to be in conjunction. This may involve two members of the solar system - including asteroids and comets. We also speak of conjunctions between these objects and stars.

Despite the fact that the planets (and the Moon) move in orbits which do not fall far outside the plane of the Earth's own orbit, really close conjunctions between planets are rare. The sky appears as a vast area and the planets themselves subtend small angular diameters.

Dia. 1 Sky looking south at 22h 00m UT (23h 00m BST) on August 1. Click the image for enlargement

Dia. 2 Constellation Lyra showing the position of epsilon in relation to the bright star Vega. Epsilon is a little over three apparent Moon diameters from Vega. Click the image for enlargement

The Sun and the Moon may appear substantially large but each only occupies 1/210,474 of the sky; in other words, 210,474 full moons would be required to cover the entire sky. (Remember we can only observe one hemisphere from the Earth's surface for which half this number of full moons would be required.) When close to the horizon both the Moon and the Sun may appear large but 720 suns (or full moons) would be required to stretch right around the horizon.

It surprises many people to find that the Moon very rarely passes close to or in front of a bright star or planet. (See archives for the occultation of Venus by the Moon on May 21 2004.) But when one considers how few bright stars there are (a mere twenty in the entire sky brighter than magnitude 1.5) the fact is not at all surprising.

The stars most favourably placed for occultation by the Moon will be those situated close to the ecliptic (the constellations of the zodiac and the constellation Ophiuchus). The first magnitude stars most likely to be occulted are Regulus (at present lying close to the ecliptic), Spica, Aldebaran and Antares. Occultations of the two latter take place quite frequently - sometimes two or three times a year from the same observation site.

Close conjunctions (or appulses) between planets and bright stars happen occasionally. For example, on 10 September 2004 the planet Mercury will pass to within 3' 12" (192") of Regulus. This is very near the resolution of the human eye and will present something of a challenge for the naked eye observer.

The "standard" test for human eye resolution of two point sources is to be found in the constellation Lyra in the form of the double star epsilon. The two stars are of almost equal brightness (magnitude 4.6 and 4.7) and are separated from each other by 207".

Lyra is well placed for observation in our northern summer skies coming to the meridian in the late evening throughout August. (See Dia. 1 & 2.)

The Mercury/Regulus conjunction will be more difficult since in addition to being closer together Mercury will be some five times brighter than the star. More details will be given closer to the event.

JV 29/07/04