Orwell Astronomical Society (Ipswich)

Summary of Lunar Occultations for 2007

More about occultations.

Introduction

During 2007, there are over 600 lunar occultations potentially observable from East Anglia, although many involve faint stars. During the year, the Moon occults the Pleiades four times, and there are three grazing occultations and three planetary occultations visible from the region.

This article summarises the circumstances of the best occultations during the year. It provides details for the location of Orwell Park Observatory; differences will in general be negligible for locations throughout East Anglia.

Occultations of Bright Stars

Table 1 lists occultation events during the year, of stars down to magnitude 5.0, where the circumstances are favourable. These events should be readily visible in small telescopes or binoculars.

The first two columns of the table list the date and time (UT) of the occultation. Column three specifies the phenomenon: "D" denotes a disappearance and "R" a reappearance. The table lists circumstances of D and/or R as dictated by the visibility of each phenomenon (determined by altitude, lunar phase, etc). Column four details the lunar phase (positive waxing and negative waning). Columns five and six give the altitude of the Sun and the star, both in degrees. (A negative solar altitude means that the Sun is below the horizon.) Columns seven and eight provide the star's magnitude and catalogue number.
 

Table 1. Occultations of stars of magnitude 5.0 or brighter.
 

Occultation Seasons

The Moon’s orbit is defined by a range of periodicities, both short and long term. The short term periodicities mean that the Moon’s path through the sky follows a pattern whereby it almost repeats itself every month. The longer term periodicities gradually shift the orbit so that no particular pattern of approximate repetition can last more than a few years. This results in so called "occultation seasons", lasting for months or years, during which particular stars are repeatedly occulted, or repeatedly not occulted.

The Moon can occult four first magnitude stars, namely Aldebaran, Spica, Antares and Regulus. During recent years, there have been no occultations of any of these stars. However, a brief occultation season of Regulus begins in 2007, during which the star is occulted twice (30 March and 07 October) – this particular occultation season does not persist beyond 2007. An occultation season of the Pleiades began on 04 December 2006 and continues through to mid-2009; during 2007, the Moon occults the Pleiades four times, on 23 February, 07 August, 27 October and 21 December. Of these, the first two occasions offer the best observing opportunity, in terms of a slender lunar crescent and a large number of occultation events. (The occultation of 17 Tau (Electra) on 27 October is visible as a graze from the North Sea a few miles off the coast at Frinton.)

Nights With Many Occultation Events

During the year, the Moon traverses some rich star fields. When this happens, a large number of occultations can occur during a single evening. Table 2 lists all evenings throughout the year when the Moon occults 12 or more stars. Nights with very large numbers of occultations are associated with the Moon’s passage through rich star fields as follows: the Pleiades on 23 February; the Milky Way in East Gemini on 21 April and 19 May.
 

Table 2. Nights with 12 or more occultations.
 

Occultations of Planets

During 2007, there are two occultations of Saturn and one of Venus. Table 3 provides details; timings refer to the centre of the planetary disk. The interpretation of the columns in the table is the same as that of table 1 (with the obvious difference that the data refers to a planet rather than a star). The first occultation of Saturn occurs at nighttime, while the second begins with the Sun above the horizon and ends after the Sun has set. The occultation of Venus is a daytime event and will therefore be challenging.

Note that the BAA Handbook for 2007 lists, in addition to the above, two lunar occultations of Uranus, on 18 February and 12 May. However, for the first the planet is at an altitude less than 2°, while the second is in fact a daytime close approach, the Moon’s limb passing within 6% of a lunar radii of the planet with the Sun at an altitude of 19°.
 

Table 3. Occultations of planets.
 

Grazing Occultations

The tracks of three grazing occultations cross East Anglia during the year. Table 4 summarises the circumstances. Columns one and two give the date and time of the graze and column three specifies the lunar limb involved. Column four indicates the distance between Orwell Park Observatory and point X on the track, on land, where the distance between the two is least; column five links to a plot of the lunar limb profile visible from X. Column six specifies a north or south displacement of the observer's position, relative to the track, based upon inspection of the limb profile, to maximise the chances of witnessing multiple disappearance and reappearance events. Column seven links to an on-line Google Earth plot of the track and column eight links to an off-line image of it. (The plot and image illustrate the nominal track, with no displacement applied.) Column nine details the lunar phase (positive for waxing, negative for waning) and column 10 the altitude of the Sun (a negative solar altitude means that the Sun is below the horizon). Columns 11 and 12 provide the horizontal co-ordinates of the star (azimuth in the order N→E→S→W). Columns 13 and 14 specify the star and its magnitude.
 

Table 4. Grazing lunar occultations.
 

James Appleton

Original: Newsletter January 2007
Updated: 26 March 2023