Lunar Occultation Of Saturn,
02 March 2007

Introduction to lunar occultations.

On 02 March 2007, the Moon occulted Saturn. Predicted circumstances for Orwell Park Observatory were as follows (event times refer to the centre of the planetary disk):
 

Table 1. Predicted circumstances of lunar occultation of Saturn.

The predictions indicated that Saturn would disappear behind the lunar limb approximately 4°W of the S Pole, in the crater Cabeus. The relatively short duration of the occultation and the southern limb aspect promised a view of Saturn appearing to slide behind features on the lunar limb at a glancing angle. (Indeed, from further west in the UK the phenomenon was visible as a grazing occultation.) This encouraged several members of OASI to attempt observations. Their observing reports are below.

James Appleton Observing From East Ipswich

I observed the lunar occultation of Saturn using a Meade 250 mm SCT with 18 mm eyepiece. The sky was a little hazy, but reasonably transparent - the magnitude 4.0 star Eta Corvi (at an altitude of only 20°) was easily visible to the naked eye despite the almost full Moon. Predictions indicated that first contact of the disk of Saturn against the Moon would on the dark lunar limb (only 10° from the bright cusp). However, in practice, undulations on the terminator meant that first contact occurred against an illuminated limb and, in the eyepiece, the planet appeared to move east to west along the limb, behind bright and dark features, before finally disappearing. Event times (UT), estimated using a stopwatch set to the speaking clock, are below.

At 02:00 UT, I completed setting up and aligning my telescope. At first it took a few seconds to locate Saturn by naked eye against the glare from the Moon. In the telescope, Saturn was visible along with Titan and a slightly fainter field star closer to the Moon. I thought I glimpsed a faint moon of Saturn close to the following ring ansa, but was unsure - later investigations with the planetarium program Redshift 5 indicated that this may have been Mimas. Glare from the Moon was a problem, although by adjusting the amount of the lunar disk in the field of view and the orientation of the eye in relation to the eyepiece, it was possible to minimise problems.

Thirty minutes later, the temperature of the telescope had fallen close to ambient, and definition was much improved. The Cassini division in Saturn's rings and a band on the planet's surface were easily visible. Saturn appeared much closer to the Moon and I could not see Titan due to glare from the Moon. The colour contrast between Saturn and the Moon was most pronounced: the Moon appeared dazzlingly white, whereas Saturn in contrast appeared dusky and pale.
 

	02:37:46	First contact.
	02:37:57	Centre of ring (in front of planetary disk) appeared to touch Moon.
	02:40:09	Second contact.
	02:57:04	First ring ansa reappeared from behind Moon. (Did not notice until several seconds after initial reappearance.)
	02:58:11	Reappearance of point on Saturn's limb where ring segment passing in front of disk meets limb of planet.
	02:59:50	Fourth contact.
	03:00:35	Second ring ansa reappeared from behind Moon.

I made a sketch of the event at the eyepiece and took some photographs. The sketch is corrected to give a true naked-eye orientation; it shows the movement of Saturn from W to E along the lunar limb during the disappearance event. The photographs were taken with a Minolta Dynax 500 Si camera in afocal mode with 200 ASA print film, using a range of exposure times from 0.5 to 2.0 seconds. Images are below.

Sketch of occultation.

Lunar topography in region of disappearance event, 02:25 UT.

Same image as preceding, with brightness boosted to make Saturn visible.

Saturn shortly after reappearance, circa 03:05 UT.

Mike Harlow Observing From Newbourne

Observed the occultation and acquired images using my 34 cm F/4.0 Newtonian with 2x Barlow and Starlight Xpress MX916 CCD. Used exposure times of ~1/100 s to register Saturn but this resulted in a very overexposed lunar image. By processing the images so that both Saturn and the lunar limb were visible it was apparent that Saturn passed behind the illuminated central peak of the crater Cabeus shortly before disappearing.

Martin Cook Observing From East Ipswich

Set alarm for 02.25 UT and started setting up 250 mm Dobsonian at 02.35 UT. Moon and Saturn were easy to find. Titan was the only moon of Saturn visible. The planet itself looked rather washed out/low contrast due to the glare of the full moon. I recorded the event times detailed below. By the time of reappearance I was cold and tired through driving the telescope manually!
 

	02:37:49	First contact.
	-	Titan occulted shortly before second contact, but failed to record timing due to need to reposition telescope.
	02:39:57	Second contact.
	02:57:22	Third contact.
	03:00:54	Fourth contact.

Ted Sampson Observing From Offton

Enjoyed very good views of the event. Set up my equipment at 23:00 UT: Meade LX 50 (200 mm Schmidt-Cassegrain) and 8x42 binoculars on a stand. Saturn looked good. Still good at midnight. Went to bed for two hours and resumed observations at 02:00 UT. View still good, but signs of dew on objective of LX 50 despite dew cap. Observed initial contact at 02:38:18 UT. Alas dew then took over and, although the Moon was visible, Saturn's reappearance was not! The binoculars however, which I had covered with a cloth for the two hours during which I was in bed, showed Saturn when several arcsec clear of the lunar limb. I regretted not covering the LX 50 similarly! All in all, a good session.