Timing lunar occultations from New Zealand

A page on occultations has lived at this address since 2005. It has been rewritten to reflect how these events are observed and timed today.

Occultation timing is one of the few areas where a backyard observer with modest equipment still makes measurements that professionals use. It is also simply exciting: a star that has been shining steadily vanishes in an instant.

What an occultation is

An occultation happens when one body passes in front of another and hides it. The commonest kind is a lunar occultation, when the Moon, moving eastward against the stars by about its own width each hour, covers a star or planet. Because the Moon has no atmosphere, a star does not fade out. It disappears almost instantaneously at the Moon's edge, and reappears just as suddenly on the other side.

Disappearances at the dark limb of a waxing Moon are the easiest to time, since there is no glare at the point where the star goes out.

Planets behind the Moon

Occultations of bright planets are spectacular. Jupiter takes a minute or more to slide fully behind the lunar limb, its moons winking out one by one before or after the planet. Such events tend to come in series, with the Moon covering the same planet month after month for a while, each time visible from a different part of the world. Saturn, Venus and Mars occultations are equally striking in a small telescope, and planetarium software will show when one is visible from New Zealand.

Why the timing matters

A precise time for a disappearance or reappearance, combined with your exact position, tells us where the Moon's edge was relative to the star at that instant. Collected over many events, those timings have helped refine:

• the profile of the lunar limb, the mountains and valleys along its edge
• the Moon's orbit and Earth's rotation
• star positions, especially before modern space astrometry
• close double stars, which betray themselves by fading in two steps rather than one

Spacecraft have since mapped the lunar limb in great detail and star catalogues from space missions are extraordinarily precise, but occultation timings remain useful, and they are the foundation of the more scientifically valuable work below.

Grazing occultations

At the northern or southern edge of the Moon's path, a star can skim along the limb, disappearing and reappearing several times behind lunar peaks. A line of observers spread across the predicted graze track, each timing their own events, can sketch the Moon's profile in remarkable detail. Grazes need travel, planning and a team, which makes them a great club project.

Asteroid occultations

The most valuable work now is timing asteroids passing in front of stars. The asteroid's shadow sweeps across Earth, typically tens to a few hundred kilometres wide. Each observer inside the path sees the star blink out for a few seconds; those outside see nothing. Plotted together, the timed chords reveal the asteroid's size and shape, and occasionally discover a small moon or a double star. Even a "miss" helps constrain the outline.

GPS-timed video

Stopwatch and voice-recorder methods have largely given way to video. A sensitive video or astronomy camera on a small telescope records the event, and a GPS-based time inserter stamps each frame with accurate time. The recording is then analysed frame by frame with free software, giving timings good to a small fraction of a second and an objective record that others can check.

Getting involved

The Royal Astronomical Society of New Zealand has an active Occultation Section that publishes predictions for New Zealand and coordinates observers across the region. Internationally, the International Occultation Timing Association supplies predictions, software and reporting standards. Both welcome newcomers, and an experienced observer will usually be glad to help with a first attempt. For finding the planets that the Moon sometimes covers, see our planets guide.