Orwell Astronomical Society (Ipswich)

Summary of Lunar Occultations for 2018

More about occultations.

Introduction

During 2018, there are almost 600 lunar occultations potentially observable from East Anglia, although many involve faint stars. The tracks of three grazing occultations cross the region during the year. No lunar occultations of planets are visible from the region during the year.

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 6.0, where circumstances are favourable. The 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 6.0.
 

Occultation Seasons

The Moon’s orbit is defined by a range of periodicities, both short and long term. The short term periodicities cause the Moon’s path through the sky to follow a pattern whereby it almost repeats 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. In 2018, the phenomenon is evident through repeated occultations of 91 Aqr.

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 10 or more stars. The large numbers of occultations on 19-21 April are associated with the passage of the Moon through the rich star fields of Taurus, Orion and Gemini.
 

Table 2. Nights with 10 or more occultations.
 

Grazing Occultations

The tracks of nine 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 the closest point on the track, on land. Column five links to a plot of the lunar limb profile visible either (a) from longitude 1.25° E (WGS84) on the track, if this is on land or, if it is not, (b) from the most easterly point on the track on land. 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 the track. (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.
 

Table 3 lists many more observing opportunities than has been the norm in previous years. In part, this is due to:

1. A graze for which circumstances in general are not ideal but the track passes close to Ipswich, meaning that a lengthy journey to a distant observing site is not required: ZC1374 on 23 April.
2. Two grazes where the Sun will have only just set, but the star is relatively bright so should be visible: μ Ceti on 21 February and ψ¹ Aqr on 04 July.

James Appleton

Original: Newsletter January 2018
Updated: 26 March 2023