Orwell Astronomical Society (Ipswich)

Lunar Occultations

Introduction

Each day, the Moon travels approximately 13° along its orbit, moving from west to east in the sky. This motion causes it occasionally to occult background stars: the eastern limb of the Moon, as it advances through the sky, obscures the star and, some time later, the star reappears from behind the western limb. Because the Moon has no atmosphere, generally disappearance happens suddenly with no preliminary fading and reappearance is equally sudden. Two exceptions to this rule occur in the case of stars with large angular diameter and stars composed of two or more close components.

The Moon's motion is confined to a strip of the sky centred on the ecliptic (the plane of the Earth's orbit around the Sun) with a width of ±6.75°, referred to as the zodiacal band. The following extract from a star chart illustrates the limits of the zodiacal band as it passes through the constellation Taurus. The sizes of stars in the map are exaggerated: in reality they appear simply as points of light. The Moon is shown to scale inside the small square: note how small it appears in relation to the piece of sky illustrated. The arrow (to scale) indicates the extent of the Moon's average daily motion.

There are approximately 850 naked eye stars (magnitudes down to 6.0) which the Moon can occult. Glare from the Moon itself tends to obscure the stars, especially when the lunar phase is close to full, and frequently necessitates use of binoculars or a telescope. However, if the Moon is very young or very old and the star is of first or second magnitude, it is sometimes possible to observe an occultation with the naked eye. Table 1 summarises the brightest stars subject to lunar occultations and the equipment typically required to observe. Among those listed are four first magnitude stars: another first magnitude star, Pollux, lies just outside the band of stars which can be occulted.
 

Table 1. Stars in the zodiacal band subject to lunar occultations.

Timing a lunar occultation disappearance or reappearance establishes a relationship between the following quantities:

• The position of the Moon.
• The local topography of the lunar limb.
• The co-ordinates of the star.
• The observer's location (determined by his topographic co-ordinates and the rotation of the Earth).
• The observer's personal reaction time (if making the observation visually).

Until the latter half of the 20^th Century, professional astronomers observed and timed lunar occultations in order to improve theories of the lunar orbit, to refine knowledge of the lunar limb and to study the rotation of the Earth. Observations of occultations also sometimes revealed errors in the position or proper motion of stars and the existence of unsuspected double or multiple stars. Nowadays, advances in determination of the lunar orbit, lunar mapping and stellar astrometrics have rendered lunar occultations largely redundant from the point of view of professional astronomers. However, they continue to provide an interesting and rewarding spectacle for amateur observers.

Visibility

The ability of an astronomer to observe an occultation depends on the following parameters:

• Moon waxing or waning. When the Moon is waxing (phase increasing) the leading limb is unlit. This provides good contrast against the star as the limb approaches and eventually passes in front of it. Conversely, a waning Moon is best for observing the reappearance of the star after an occultation, since the following limb of the Moon is then unlit.
• Age of the Moon. A young Moon is best for observing occultations, as glare is less of a problem. When the Moon is young, earthshine provides faint illumination of the dark portion, enabling the observer to prepare for the moment of occultation.
• Magnitude of the star. The brighter the star the easier it is to observe against the Moon's glare. A brighter star therefore generally enables the observer to make a more confident timing.

Lunar Occultations Of Planets

The Moon occults planets as well as stars. In fact, all the planets with the exception of Venus are confined within the zodiacal band of ±6.75° defined by the Moon's motion. Venus is the exception, and can stray up to ±9.8° from the ecliptic. Occultations of the brighter planets are visible to the naked eye; the fainter planets will require a telescope.

For a given location on the Earth's surface, a lunar occultation of a major planet will occur almost every two years on average. However, the occurrence is very irregular, with clusters of events separated by lengthy periods with no events.

Occultations of planets may be predicted in broadly the same way as occultations of stars. However, the motion of the planet needs to be accounted for in calculating the details.

Grazing Lunar Occultations

If the Moon passes centrally in front of a star, it can obscure the latter for more than an hour. However, generally the centre of the lunar disk passes to the north or south of the star, causing a shorter obscuration. In the limit, the north or south polar limb of the Moon briefly obscures the star - this case is termed a grazing lunar occultation or graze. During a graze, mountains and valleys on the lunar limb can pass in front of the star, alternately obscuring and revealing it, making it appear to flash off and on. Such an event is typically visible over a strip of land on the Earth's surface only a few hundred metres wide, referred to as the graze track. On one side of the graze track, an observer will witness a proper occultation, while on the other side he/she will witness a "near miss". In an average year there are a dozen or so grazing occultations that can be observed from the British Isles. Generally, a south polar graze is more spectacular than a north polar graze because the Moon’s southern limb is the more rugged of the two.

Prediction Of Lunar Occultations

Circumstance of occultations, which are not grazes, for the location of Orwell Park Observatory are predicted as follows:

• The NASA JPL (Jet Propulsion Laboratories) reference ephemeris DE-430 [2] provides the position of the Moon.
• The following star catalogues are used, in order of preference, for stellar positions: (1) Hipparcos [3]; (2) Tycho-2 [3] (with proper motion data from ACT [4] where available); (3) PPM [5]; and, finally, (4) XZ94D. (Hipparcos provides stellar positions accurate to sub-milliarcseconds (mas) to magnitude 9.0 and to 25 mas for fainter stars. Proper motion (pm) data for Tycho-2 stars was obtained over a short timescale and is not particularly accurate, so pm data from the ACT catalogue is used in preference where available. The PPM catalogue fills some gaps in the coverage of Hipparcos/Tycho-2 and data from IOTA's XZ94D provides data for five variable stars not represented correctly in the other catalogues. Stars from the Hipparcos and Tycho-2 catalogues account for 99.8% of the total.)
• Timing corrections for the lunar limb are based on electronic Watts charts, a digitised version of the lunar limb profile published in 1963 by the American astronomer C B Watts (1889-1971) working at the US Naval Observatory. (Watts' monumental work takes the form of a 951 page book [6] detailing the elevation of the limb above or below its mean level all around the circumference, for nearly all libration angles. The tome was based on 475,000 measurements made during a period of 20 years of 700 photographs of the Moon. Unfortunately, the data is of variable quality and is known to have several significant deficiencies.)
• First a fast, approximate search for potential occultations is carried out within a narrow band of the sky centred on the ecliptic.
• For each potential occultation found as above, the circumstances ar re-calculated accurately and, if an occultation occurs, observational details are listed. (The calculations are performed in the fundamental plane - the plane running through the centre of the Earth perpendicular to the line to the star. An occultation occurs when the projections of the Moon and the observer overlap in the fundamental plane.
• If an occultation occurs, checks are run to determine whether the circumstances permit observation from Orwell Park Observatory (star at sufficiently high altitude and sufficiently bright in relation to lunar phase, Sun sufficiently below horizon, etc.) If so, a correction is applied to predicted event times to allow for the local topography of the lunar limb. Typically, this can make a difference of up to a few seconds.

The occultation package Occult by the Australian amateur David Herald, is used to calculate graze predictions from 2014 onwards. For earlier years, predictions are based on the software programme graz written by Jean Meeus, enhanced to use the NASA JPL ephemeris DE-405 or DE-430 and the star catalogues listed above together with electronic Watts charts. Occult enables a much more accurate approach primarily through providing access to lunar limb data from the 2007 Japanese Kaguya (officially Selene) mission; this significantly improves the overall accuracy of graze predictions.

Occultation predictions for Orwell Park Observatory.

Predictions of occultations may be found in the monthly astronomy magazines and on the websites of the British Astronomical Association and International Occultation Timing Association.

References

James Appleton

Updated: 26 March 2023