Lunar Occultation Highlights for 2003

Moon occults Spica
Spica (upper left) grazes the Moon's north polar region early in the morning of November 30, 1994.
Courtesy Motomaru Shirao.
Nothing else visible in the universe beyond our planet happens so fast. Perhaps you’ve been watching in a small telescope as the edge of the Moon creeps up on a star. The tiny point of light appears to sit on the Moon’s limb for a few seconds, and then "poof" — it’s gone! Just as abruptly, about an hour later, the star reappears on the Moon’s other side. The startling suddenness of these events proves that stars have tiny angular sizes — smaller than could be measured any other way until recently.

All skywatchers should mark their calendars to watch the best of these occultations. Even more satisfying is timing them accurately, a simpler task than many observers think. Very accurate timings can often be made by just pointing a camcorder into the eyepiece of your telescope. Some stars are so bright that you can zoom in on the Moon with the camcorder directly — you don’t even need a telescope. (More information is available in the article "Camcorder Timing Tips".)

Once again we at the International Occultation Timing Association (IOTA) provide information about the events visible throughout the world in the coming year. The emphasis is on those that can be seen from North America, Europe, Australia, and New Zealand, which are populated with many active observers. Only the brightest events can be mentioned in this article; more detailed information for these and other areas is available via the Internet (see page 7).

This Web article is a summary of the longer feature found in the January 2003 issue of Sky & Telescope. Included in the magazine article are tables with details of bright-star occultations for North America, Europe, eastern Australia, and New Zealand, as well as information about observing occultations remotely, tips on timings, and a description of a well-observed grazing occultation.

Background Details

Occultation Map
When the Moon passes in front of a bright star or planet, observers in some places on Earth (but not others) can see the light of the more distant body cut off. This map shows the year's best events worldwide (click on the image to see the complete chart); detailed maps later in this article give many more. A blue line denotes the northern limit of an occultation's visibility region, where the star will appear to graze the Moon's northern limb and perhaps blink off and on several times behind lunar mountaintops. A red line indicates an event's southern limit. Ticks are spaced 10 minutes apart, with the Universal Time shown on only a few of the ticks to avoid crowding. Time increases to the right.
S&T illustration.
In 2003 the Moon will cover Mars, Venus, and Mercury, as well as many 2nd- to 4th-magnitude stars. It will often pass over the large open cluster NGC 1746 in Taurus, and it will occult numerous faint stars during two total lunar eclipses. It is important to realize that a star’s disappearance or reappearance can occur up to two hours earlier or later than the stated hour, depending on your location.

When the Moon is waxing, on its way to full, stars almost always disappear on the Moon’s dark limb and are easy to watch. The reappearance happens on the bright limb, which overwhelms most stars with glare and makes them too hard to watch or time. When the Moon is waning, the events on the bright and dark limbs are reversed. Reappearances take more care, since you need to be looking at the part of the Moon’s limb where the star will emerge. To anticipate the Moon’s brightness and whether the star will enter or leave at the dark limb, the lunar phase (in two tables on page 6) is listed as the percentage of the Moon’s disk that is sunlit followed by "+" for waxing phases and "–" for waning phases.

A grazing occultation is visible within a mile or so (2 to 3 kilometers) of an occultation’s predicted northern or southern limit. There you might see the star wink off and on several times as it passes behind hills and valleys near the Moon’s north or south pole. Observers spaced across this path will time different sequences of events, which can be analyzed to map the lunar-limb profile. Visual timings are fine for grazes, since their scientific value depends more on knowing each observer’s geographical location than on obtaining timings to better than, say, ½ second. Grazes are important for mapping the Moon’s apparently ice-rich polar regions, because the laser aboard the U.S. Naval Research Laboratory’s Clementine spacecraft did not reach those areas during its 1994 mapping mission from lunar orbit.

Grazes are the most dynamic, interesting, and valuable kind of lunar occultation. Many require a small telescope. but a camcorder alone, or firmly mounted binoculars, might suffice for the brighter ones.

Occultations of Bright Stars for North America in 2003

Occultation Map of North America
An occultation can be seen north of a red line or south of a blue one. A grazing occultation occurs along each line; tick marks at 10-minute intervals (increasing west to east) indicate the Universal Time of the middle of the graze. A circled A means the event is visible but the Moon’s altitude is too low for reliable timings, S means sunrise or sunset for bright objects, and T shows where twilight becomes too strong. B marks where a central graze occurs at the Moon’s north or south cusp and terminates the path (unless the star is bright enough that it might be observed against the sunlit side of the Moon).
S&T illustration.


Occultations of Bright Stars for Europe in 2003

Occultation Map of Europe
An occultation can be seen north of a red line or south of a blue one. A grazing occultation occurs along each line; tick marks at 10-minute intervals (increasing west to east) indicate the Universal Time of the middle of the graze. A circled A means the event is visible but the Moon’s altitude is too low for reliable timings, S means sunrise or sunset for bright objects, and T shows where twilight becomes too strong. B marks where a central graze occurs at the Moon’s north or south cusp and terminates the path (unless the star is bright enough that it might be observed against the sunlit side of the Moon).
S&T illustration.


Occultations of Bright Stars for Australia and New Zealand in 2003

Occultation Map
An occultation can be seen north of a red line or south of a blue one. A grazing occultation occurs along each line; tick marks at 10-minute intervals (increasing west to east) indicate the Universal Time of the middle of the graze. A circled A means the event is visible but the Moon’s altitude is too low for reliable timings, S means sunrise or sunset for bright objects, and T shows where twilight becomes too strong. B marks where a central graze occurs at the Moon’s north or south cusp and terminates the path (unless the star is bright enough that it might be observed against the sunlit side of the Moon).
S&T illustration.


Special Events for 2003

Crescent Moon and Mercury
The crescent Moon about to occult Mercury on the morning of June 14, 1996.
Courtesy Akira Fujii.

Planets. Only the inner major planets will be occulted in 2003, especially Mars — see the table below. There is probably little new that can be learned from these events in this age of space exploration, but they're still interesting to watch. The controversial "ashen light" that some observers have reported on Venus’s dark side might be observed briefly during a lunar occultation, but neither of this year’s events is favorable for attempting such an observation. The planet is mostly sunlit and relatively close to the Sun, meaning that only very narrow regions will see the planet not too low in a not-too-bright twilit sky. On May 29th the best view will be from Madagascar or eastern Kenya, while on October 26th it will be from east-central Brazil.

Occultations of Solar-System Objects, 2003
DateUT (h)PlanetMag.DiameterMoon (%)Area of visibility
Jan. 2715Mars  1.3  5.1"26-S Pacific, S S. America
May  313Iris  9.8  0.1" 3+India, W China, Nepal
May 29 4Venus-3.910.7" 4-E Africa, SE Asia, Japan
July 17 8Mars-1.919.6"85-NW S. America, Carib.
Sept. 912Mars-2.724.3"98+NE Asia
Oct.  615Mars-2.019.6"87+SE Australia, NZ
Oct. 2620Venus-3.910.4" 3+Hawaii, S S. America
Nov. 25 3Mercury-0.5  5.3" 2+Java, W & S Australia, NZ

Open Cluster Passages. NGC 1746 in Taurus is the only significant star cluster that the Moon will traverse in 2003. It contains about 60 stars ranging from magnitude 8 to 11. The dates of these and one other passage of the Moon through an open cluster are given in the table below. The magnitude refers to the combined brightness of the whole cluster and is followed by its diameter in arcminutes. For comparison, the angular diameter of the Moon is about 30'.

Occultations of Open Clusters, 2003
DateUT (h)ObjectMag.Diam.Moon (%)Area of visibility
Mar. 11 1NGC 17466.045'47+E N. America
Apr.  7 8NGC 17466.045'25+Alaska, Yukon
May  414NGC 17466.045' 8+C Asia
July 25 9NGC 17466.045'14-E & N N. America
Aug. 2117NGC 17466.045'34-NE Asia, Japan
Oct. 2912NGC 65208.1 5'21+N India, C Asia

Lunar Eclipses. During a total eclipse of the Moon, occultations of 8th-magnitude stars are easy to time. Even those of 9th and 10th magnitude can be seen against the lunar limb except in the brightest outer parts of the umbra. Both of 2003’s lunar eclipses (on May 15–16 and November 8–9) are visible from the Americas and from parts of Europe and Africa. The southern limit for the (May 16th) eclipse occultation of 5.5-magnitude ZC 2217 in Libra is shown on the North America map.

For More Information

If you're interested in timing occultations, be sure to refer to the article "How and Why To Make Occultation Timings" elsewhere on this Web site. Predictions of lunar occultations for 17 North American stations are given in the 2003 Observer’s Handbook of the Royal Astronomical Society of Canada. About 30 occultations of stars as faint as 5th magnitude are listed for each location. The handbook also has maps of northern and southern limits for 256 occultations of stars to magnitude 7.5 for the U.S., Canada, and northern Mexico.

IOTA’s Walt Robinson (515 W. Kump, Bonner Springs, KS 66012-1439) will compute occultation data if you send him accurate geographical coordinates and an e-mail address or a long, self-addressed, stamped envelope.

Observers from Europe and the United Kingdom are encouraged to join IOTA/ES. European residents should contact Hans Bode, Bartold-Knaust Strasse 8, D-30459 Hannover, Germany.

The International Lunar Occultation Centre in Tokyo collects and analyzes timings; reports can be sent to them at iloc@jodc.go.jp using forms available from IOTA’s Web site.

Annual membership in IOTA costs $30 in North America ($35 overseas) and includes free graze predictions for stars brighter than 9th magnitude, local circumstances for the approaches of asteroids to stars, descriptive materials, and a subscription to the Occultation Newsletter (available separately for $20, more overseas).

Also see this Web site's Occultations section, which contains informative articles about upcoming occultations and how to observe them.