When Jesus was born in Bethlehem of Judea, in the days of King Herod, behold, magi from the east arrived in Jerusalem, saying, “Where is the newborn king of the Jews? We saw his star at its rising and have come to do him homage.”

–Matthew 2:1

Royal Beauty Bright

Notre Dame Astrophysicist researches the Christmas star

Sometimes an old story can be refreshed with a new perspective, even one from far out in the galaxy.

That’s what Grant Mathews, director of the Center for Astrophysics at Notre Dame, figured when he decided to use his expertise to see if he could identify the star of Bethlehem that the Bible says announced the birth of Jesus.

“I’m an astrophysicist, so people see things in the sky, and I make models that explain them,” Mathews says. “It’s Christmastime, and here’s this astronomical event that’s been recorded, and I decided to just look at it as a scientific question.”

Mathews began his research with three basic questions:

When did it occur?

Scripture refers to the reign of Herod the King. Roman historian Flavius Josephus recorded Herod’s death as after a lunar eclipse, with his burial before the first day of spring. The Book of Luke says John the Baptist began preaching in the 15th year of Emperor Tiberius. Since John and Jesus were born six months apart and Jesus began teaching around age 30, these references point Mathews to a time frame of 8-4 B.C.

Did anyone else see it?

Chinese astronomers kept careful records of events in the sky going back beyond 1000 B.C. They record Halley’s comet in 12 B.C. and several astronomical phenomena in the right time frame, including comets, novae, supernovae and planetary alignments.

Timeline of astronomical events around the birth of Jesus
A - Halley’s comet
B - Comet with tail
C - Comet, no tail or motion. Possibly a nova or supernova.
14 B.C. 12 10 8 6 4 2 B.C. Halley’s comet Birth of Jesus Comet with tail Comet, no tail or motion. Possibly a nova or supernova.

What did it look like?

And behold, the star that they had seen at its rising preceded them,

indicates a morning star

until it came and stopped over the place where the child was.

indicates a second movement, a stop

–Matthew 2:9

Mathews went back through NASA records that can estimate backward to astronomical events in the past. This is possible because events like comets and planetary alignments occur at regular intervals that can be traced backward, and novae and supernovae leave remnants behind.

Mathews ruled out a comet, nova and supernova because these events were feared in ancient times as harbingers of disasters rather than positive signs.

  • Comet

    A celestial object — consisting of ice, frozen gas and dust — that has an elliptical orbit and forms a long tail when near the sun.

  • Nova

    A star that suddenly becomes many times brighter before returning over months or years to its original intensity.

  • Supernova

    The explosion of a star that has reached the end of its life, causing a sudden burst of light that can be seen for a while as it travels across space.

  • Planetary Alignment

    The sun, earth and at least one planet lie in a straight line.

The magi, who were Zoroastrian priests from Babylonia or Mesopotamia, would not have followed bad omens, eliminating the first three options. But a special planetary alignment that looked like a star could have motivated their journey. It also would explain why no one else in the region noticed, likely because it wasn’t enormously bright.

Zoroastrian priests believed that each constellation in the sky was connected with the motions of the sun, moon and planets. Each constellation of the zodiac was assigned to a different geographic region of influence, as well as speaking to the character of a person born that day. For instance, Aries (the Ram) was associated with Judea and Palestine (Herod’s kingdom).

Herod’s Kingdom was assigned to Aries, whichincluded Judea and Palestine.

So Mathews studied planetary alignments that occurred in his target time frame of 8-4 B.C. There was a very special alignment on April 17, 6 B.C. At that time, the sun, moon, Jupiter, Saturn and the vernal equinox were all aligned in the Aries constellation, while Venus aligned in nearby Pisces, and Mercury and Mars were on the other side in Taurus.

This alignment is so unusual it is not expected to occur again until the year A.D. 16213 — and even then not in Aries or the vernal equinox, which is the place in the sky where the sun crosses the projection of the equator.

What significance would this planetary alignment have for the magi?

PISCES Saturn Equator projection Vernal equinox Venus Jupiter Moon ARIES Sun Mars Mercury
  • Sun – a significant person would be born in Judea (Aries)
  • Moon – a person with a special destiny to die at an appointed time
  • Jupiter – a ruler, very powerful when with the sun
  • Saturn – a giver of life
  • Vernal Equinox – new life
  • Other planets – strengthen importance

The importance of April 17, 6 B.C., is suggested by a coin minted in Antioch to commemorate the unification of Syria and Judea that depicts a bright star.

The star on the Roman coin corroborates the identification of Aries with Judea.
M. Molnar, “The Star of Bethlehem”

Mathews also believes his theory explains another mystery of the Christmas story. The story says the star preceded the magi during their journey and then stopped over the place of Jesus’ birth.

In astronomical terms, Mathews said planets usually appear to move from west to east relative to the background stars. But in a phenomenon known as “retrograde” motion, the outer planets appear to change to a westward motion when the Earth, with its shorter orbit, overtakes the outer planets. About six months later, Jupiter would appear to stand still as it reversed its motion to eastward again.

Direction the sky moves Path of Jupiter
Jupiter in retrograde motion

As an astrophysicist, Mathews said he feels a kindred connection to the magi astrologers of old.

“Today, we look at the sky … we’re looking for the truth revealed in the heavens,” he said. “And in that case, in that time, I believe God honored that sincere attempt by those viewers of the heavens of this pending event.”

A beige sketch of three wise men pointing up at a star.

For more information, listen to the Notre Dame Stories podcast episode, “What was the Christmas Star?