Orwell Astronomical Society (Ipswich)

Summary of Lunar Occultations for 1998

More about occultations.

Introduction

During 1998, there are approximately 640 lunar occultations potentially observable from East Anglia, although many involve faint stars. There are two grazing occultations of stars visible from East Anglia. The Moon does not occult any planets during the year as seen 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 brighter than 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 brighter than magnitude 4.0.
 

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 effect is most pronounced for the four first magnitude stars that the Moon can occult, namely Aldebaran, Spica, Antares and Regulus. We are currently in the middle of an occultation season of Taurus and, during the year, there are three occultations of Aldebaran and several passages of the Moon through the Hyades star cluster.

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 more than 10 stars; many are associated with the Moon’s passage through rich star fields in Taurus.
 

Table 2. Nights with more than 10 occultations.
 

Grazing Occultations

The track of two grazing occultations cross East Anglia during the year. Table 3 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 3. Grazing lunar occultations.
 

James Appleton

Original: Newsletter January 1998
Updated: 26 March 2023