A snowy landscape, carollers bundled up in hats and scarves, a tree strung with colourful lights.

A classic Christmas scene is hard to fault, but according to one astronomer most holiday images are completely off the mark when it comes to one detail: the moon is often drawn in the wrong lunar phase.

Dutch scientist Peter Barthel, from the Kapeteyn Astronomical Institute in the Netherlands, is on a quest to correct the scientific mistakes on Christmas cards, children's books and even wrapping paper.


Phases of the moon. (Brett Lamb/iStock)

In his paper "Santa and the Moon" published earlier this year in the journal Communicating Astronomy with the Public, Barthel examined illustrations in several storybooks, wrapping paper and Christmas cards he collected in the U.S. and the Netherlands.

He found that illustrators and designers were drawing moons without regard for how the lunar cycle actually works.

The most common mistake he noticed was the depiction of a waning crescent moon (also known as a third or last quarter moon) — which is only visible in the early morning — in scenes that take place in the evening.

"You see children hanging around with Christmas trees, or you see children decorating their houses for Christmas, or you see St. Nicholas distributing the presents, and we know this is something that this guy does in the evening," said Barthel.

Phases of the moon

The moon takes about 29.5 days to orbit the Earth. Depending on where it is in this orbit, the moon reflects the light of the sun to varying degrees – and different parts appear visible to us on Earth:

  • New - Not visible
  • Waxing crescent - Less than half is visible; illuminated part is increasing
  • First quarter - Half is visible; illuminated part is increasing
  • Waxing gibbous - More than half (but not fully) visible; illuminated part is increasing
  • Full - Rises at sunset; fully visible all night long
  • Waning gibbous - More than half (but not fully) visible; illuminated part is decreasing
  • Last quarter - Half is visible; illuminated part is decreasing
  • Old crescent - Less than half is visible; illuminated part is decreasing

Source: Farmer's Almanac

"There are obvious evening scenes, which should only have a first quarter moon or a full moon to be scientifically accurate."

Barthel has also noticed astronomical mistakes in popular culture during other parts of the year, especially around Halloween.

"The are horror movies, for instance, where you hear a clock striking twelve and then you see behind the horizon, a full moon rising," said Barthel. "But that's not possible because a full moon rises at the moment the sun sets."

Barthel said that after his study was published, there was a backlash from people who called his work ridiculous and a waste of time and money.

But for Barthel, the goal of his research was to use the holidays as an opportunity to teach the public about basic astronomy and how the lunar cycle works. 

He says that it is his job as a scientist is to capture people's attention with a strange or unusual factoid and then use these examples to explain and educate.

It already seems to be working. Barthel says that he has received letters and emails from Christmas card illustrators who want to understand their scientific mistakes and correct them.

"My recipe for science communication is very simple," Barthel said. "It's Wow! and Aha!"