"Two lights are better than one - right?' This is what you get during a conjunction - two or more objects, e.g., planets appear very close together, or even overlap, on the sky, and together form a very bright object. Several astronomical events involving conjunctions of planets occurred around the time of Jesus' birth: On May 19, 3 B.C., Saturn and Mercury were in close conjunction - within 40 minutes of arc of each other. (For comparison, the width of a full moon is about 30 minutes of arc). Then on June 12, 3 B.C., Saturn moved eastward across the sky very close to Venus. During this conjunction the two planets were about a quarter moon diameter apart.

Two months later Jupiter (the 'Planet of Kings') joined the action and came into close conjunction with Venus (the 'Mother Planet') just before sunrise, coming within 4.2 minutes from each other as viewed from earth, and appearing as a bright morning star. And if there wasn't enough going on in this short period, on June 17, 2 B.C., Venus and Jupiter joined yet again, this time in the constellation Leo the Lion (which also contains Regulus - the 'Star of Kings'), when the disk of Venus actually passed in front of Jupiter, occulting it partially. These objects would again have appeared as a single brilliant star! In fact, there has not been a brighter, closer conjunction of Venus and Jupiter in Leo so near to Regulus in the 2,000 years before or since. Marvellous and plausible as the conjunction theory is, for this explanation to be true Herod must have died at a later date than is commonly believed (in 4 B.C.).

Retrograde motion of Jupiter?

If we accept the 'Star of Bethlehem' event to at least involve a planet, one of the qualifications for the object still needs to be satisfied: the Bible tells that the star came to rest over Bethlehem. But how does a planet just stop? Planets can't stop in their orbits around the sun! However, to the Earthbound observer, a planet can appear to stop on the sky when it is in 'retrograde motion'. The explanation for retrograde motion arises from the fact that planets further from the sun are moving more slowly in their orbits than those closer to the sun. So, as Earth overtakes, for example, Jupiter in its orbit around the sun, Jupiter appears to slow down - stop for a period of a few days - backtrack on its path on the sky and stop again - before it resumes it path among the fixed stars.

In August 12, 6 B.C. Jupiter did just that - it appeared from Earth to have reached 'full stop' in its travel through the stars. Then it resumed its motion through Aries (the sign of the Jews) when it became stationary again on December 19, 6 B.C. Several scholars have proposed that the Bethlehem Star was indeed Jupiter passing through a stationary point in its trek across the sky. It is conceivable that the Magi viewing from Jerusalem would have seen Jupiter - the King Planet - stopped on the sky in the direction of the little town of Bethlehem. On the other hand, retrograde motions of the visible planets are not uncommon - just how many times do people expect Wise Men to jump on their camels and journey to Jerusalem?

Although we still don't know precisely what astronomical event (or possibly series of events) the 'Star of Bethlehem' adopted, there are certainly several strong contenders - a brilliant conjunction, an awesome occultation, a comet, or even a mysterious, 'stopped' planet.

As someone long interested in science and religious belief, I am intrigued by the biblical account that Herod had to ask when the 'Star' appeared. Anyone of us can glance up and see planets and stars (the planets are the ones that don't twinkle), but apparently, in Jerusalem some 2,000 years ago one could look up and see the 'Star of Bethlehem' without realising it. Herod didn't know of it! It took magi and their astrological mindset to explain it (which again makes the point that these astronomical observations must have been interpreted in astrological and cultural context).

What is certain is once the 'Star' was pointed out, and its political significance explained, all Jerusalem was abuzz, and Herod set in motion his murderous plot.

Ralph Robinson is Professor of Parasitology at the University of the West Indies, Mona, and an amateur astronomer.

Additional reading material: Highfield, R. (1998). Can reindeer fly? Metro Books. London. ISBN 1 900512 44 0. Humphreys, C. (1995) The Star of Bethlehem. Science and Christian Belief 5: 83-101.

Molnar, M. (1999). The Star of Bethlehem: The Legacy of the Magi. Rutgers University Press: ISBN 0-8135-2701-5. Star of Bethlehem http://www.bethlehemstar.net/. The Star of Bethlehem http://sciastro.net/portia/articles/thestar.htm

Was it a meteor?

A meteor, or 'shooting star', is a fragment of material which enters Earth's atmosphere at very high speed, glowing brightly as its outer layers vaporise. Although often physically tiny, a shooting star can be stunning when viewed from Earth and could be a dramatic means of making a heavenly announcement. While they do move ('shoot') across the sky, meteors don't rise in the east like other stars, and burn up in the atmosphere within a few seconds. Hence, meteors, and for that matter their larger meteorite relatives, can be ruled out as a likely candidates for the 'Star'.

An exploding star?

Chinese astronomers recorded an exploding star, or 'nova' in the constellation Capricorn in March-April of 5 B.C. which was visible for almost three months. The occasion of a nova was certainly considered by the ancients as heralding an important historical event. Technically, a nova is caused by an existing (usually dim) star acquires enough material, usually from a nearby companion, to raise the surface pressure high enough for a huge thermonuclear explosion to happen. The nova reported by the Chinese would have appeared in the east several hours before sunrise.

However, a couple of problems accompany this theory: a nova would not have moved relative to other stars - hence could not have 'gone before' the Wise Men on their trip to Bethlehem - and the Chinese records imply that it was not very conspicuous at all, hardly befitting the birth of a king!

In our Milky Way galaxy, over the past thousand years, there have been four brilliant supernovae, in 1006, 1054, 1572 and 1604. Clearly, we are long overdue for another!

An occultation by the moon?

The moon can 'occult' a planet causing the planet to disappear when it, our closest neighbour, moves in front of the planet, thereby blocking it from earthly view. A double occultation of Jupiter by the moon in the constellation Aries (the sign of the Jews) occurred on March 20, 6 B.C., and again on April 17, 6 B.C. when Jupiter was in the eastern sky. These events are considered by some scholars to be astrologically significant, so much so that they are portrayed on the aforementioned Roman coin of Antioch. Given the lofty status of Jupiter as the 'Planet of Kings', however, the blotting out of this particular entity seems less to foretell the birth of a king than the death of one.

Notwithstanding, the negative possible undertone, such occultations may have been considered truly messianic - a plausible candidate 'Star'?

Photograph by Gerald H. Newsom

Comet Hale-Bopp in 1997.

A comet?

Long ago comets, like novae, were regarded as heralding important events (though, unlike novae, comets usually foretold something awful!) Records of Chinese and Babylonian astronomers indicate the appearance of several comets around the time of Jesus' birth - in 5 and 4 B.C., but include no mention that they were bright. A comet appeared in March-April, 5 B.C. at the same time as the nova, and the possibility that this was confused with the nova, or vice versa, is a niggling one. Further, the Chinese reports imply that the object was stationary, which is quite unlike a comet that could be seen for several months.

Although the Chinese associated comets with 'sweeping away the old order of things', the general populace in the Middle East of some 2,000 years ago may not have agreed. Comets were felt to be bad omens, and many scholars hold the view that it is unlikely that a comet could have been the 'Star' that announced the birth of the Jewish Messiah, or inspired the Magi to embark on the long journey to Bethlehem.

Plot of retrograde motion of Jupiter in the constellation Cancer in 2003 (by John Rummel).