Grazing lunar occultation

A lunar occultation occurs when the Moon, moving along its orbital path, passes in front of a star or other celestial object, as seen by an observer (normally on the Earth). Light from the occulted object is blocked by the moon and a perceptible shadow of the moon is cast onto the ground if that ground is in night-time (facing away from the sun). The moon's shadow moves west-to-east due to the orbit of the moon and the rotation of the earth, as shown on the image at left.

A map showing the path of the lunar shadow as it crosses the earth. In this case, the star is Antares and the predicted date is 2027 September 7th.

The left (western) ellipse represents moonrise for that region of the globe. The right (eastern) ellipse represents moon set for that region. The northern and southern path limits are shown. In the example shown, weather conditions allowing, sites between the white lines will see the event at night; sites between the blue lines will see the event during twilight; and sites between the red dotted lines during the day-time for stars brighter than (of a number lower than) second magnitude (<+2 Apparent Magnitude).[1]

A grazing lunar occultation (also lunar grazing occultation, lunar graze, or just graze) is seen at locations along the north and south limits, and the observer will see the object disappear as the shadow of mountains pass by, and reappear as the light passes down the valleys on the edge of the moon.

Observed events plotted on the limb profile.

A plot of the results from a single observer graze expedition, where eight events were observed. The path of the star is shown curved, when in reality it is the moon moving past the star. The first disappearance is shown on the left (red) and the last reappearance on the right (green). The olive coloured dots are altitude soundings from the data points from the Kaguya lunar orbiter.

A team of many observers can combine observed events and construct an extremely accurate profile of the lunar terrain. Since graze paths rarely pass over established observatories, amateur astronomers use portable observing equipment and travel to sites along the shadow path limits. The goal is to report the UTC of each event as accurately as possible, and GPS disciplined devices are frequently used as the time-base.

Two methods are used to observe

• visual – the observer has an audible UTC beeping device (e.g. a shortwave radio tuned to WWV) and an audio recorder (e.g. a tape recorder) and watches the target through the telescope, and calls 'Gone' when the star disappears, and 'Back' when the star reappears. The audio recording is later analysed to extract the event times.
• video – the observer uses a small video camera, usually mounted in the focuser of the telescope. A Video Time Inserter (VTI) is commonly used to insert a UTC time-stamp onto each frame of the recording. Either a camcorder, DVR or a laptop computer is used to record the video stream. The video recording is later analysed to extract event times.

All known lunar occultations, (both total lunar occultations as well as grazing lunar occultations) are archived at the VizieR service.[2]

Such observations are useful for;

• refining knowledge of the positions and motions of stars[3]
• examining limb profile around the lunar polar region.[4]
• Measurement of the Earth's rotation[5]