"Luna" January 1998
Saturn's Superb Vanishing Act
On the morning of 12 November 1997 the Moon passed in front of Saturn, producing an occultation that delighted thousands of amateur astronomers in the western hemisphere who observed the event. For once, the skies over Britain were, on the whole, relatively cloudless. Saturn took about a minute to disappear behind the Moon's dark limb, and remained hidden for about half an hour, making a reappearance from the bright limb. An observational drawing sequence of the event was sent by Alan Heath (Long Eaton, Nottingham).
The next occultation of Saturn by the Moon will take place on the morning of 1 December 2001, with a near-full Moon and a disappearance behind the dark lunar limb.
Bull's Eye Sports a Lunar Patch
At around 18:40 UT on the evening of 28 April the crescent Moon passes in front of the bright star Aldebaran. The orange "bullseye" of Taurus will be hidden by the Moon until it moves out of the way around half an hour later. This series of Aldebaran occultations continues through 1998 and in 1999.
The Moon ploughs a half-degree wide swathe across the sky, moving its own diameter every hour. Since the Moon's orbital plane is inclined by five degrees to the ecliptic, all stars lying within five degrees either side of the ecliptic are eventually occulted. All of the near ecliptic planets undergo lunar occultations.
When a planet is partially hidden by a bright lunar limb, it often appears to be separated from it by a dusky line - simply a contrast effect between the two bodies, not an indication of a lunar atmosphere. If the Moon did have an appreciable atmosphere then stars and planets would appear to fade well in advance of their occultation. Because our satellite has no atmosphere - and stars have such tiny apparent diameters - most occultations happen very suddenly. But fadings or staggered effects can suggest previously unsuspected double or multiple star systems. I clearly observed such phenomena during the total lunar eclipse of 17 August 1989, where the known double star 44 Capricorni grazed the lunar limb and fluctuated in brightness, so the multiple nature of the star is indicated.
Occultation timings are important because they provide a way to monitor the Moon's motion through space and can also highlight variations in the Earth's rotation rate. Occultations may reveal errors in a star's accepted position and lead to a revision of that star's proper motion - its actual path plotted against the celestial sphere. Proper motion was first discovered in 1718 when Edmond Halley showed that the position of Aldebaran, relative to its surrounding stars, had changed since ancient times.
Star disappearances behind the unlit western limb of the Moon (at waxing crescent phase) are conveniently staged in the evening sky; a degree of anticipation is in the observer's favour, since the star can be seen to gradually approach extinction at Moon's limb, especially when there is a hint of earthshine giving away the limb's position. Re-appearances at a brightly illuminated limb require great skill to record with an equivalent level of accuracy.
Lists of stars liable to be occulted are published in numerous astronomical ephemerides. Occultations of brighter stars, like the Pleiades, Regulus or Spica, and the more visually stunning planetary occultations, are always featured in Popular Astronomy.
