Orwell Astronomical Society (Ipswich)

Summary of Lunar Occultations for 2017

More about occultations.

Introduction

During 2017, there are over 650 lunar occultations potentially observable from East Anglia, although many involve faint stars. The tracks of three grazing occultations cross the region during the year within a few kilometres of Orwell Park Observatory. 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 to magnitude 5.5, 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 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 2017, the phenomenon is evident through repeated occultations of Aldebaran and stars in the Hyades.

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 30 April is associated with star fields in Gemini.
 

Table 2. Nights with more than 10 occultations.
 

Grazing Occultations

The tracks of three 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.
 

James Appleton

Original: Newsletter January 2017
Updated: 26 March 2023