There was an early Christmas present in the southwest sky of Guam on Dec. 21.

The “Christmas Star” is visible only once every 800 years.

Also called the Great Conjunction, the Christmas Star refers to the exceptionally close planetary conjunction between Jupiter and Saturn.

According to NASA, the planets regularly appear to pass each other in the solar system, with the positions of Jupiter and Saturn being aligned in the sky about once every 20 years.

What makes this year’s spectacle so rare is it’s been nearly 400 years since the planets passed this close to each other in the sky, and nearly 800 years since the alignment of Saturn and Jupiter occurred at night, making it visible from Earth.

These planets were the brightest objects on the southwestern horizon, and passed within only 0.1 degree of each other in the constellation Capricornus. Although they may appear to be very close to the naked eye, in reality, they are hundreds of millions of miles apart.

Extraordinarily, this conjunction took place on the same day of the winter solstice, which made the planets appear brighter and made the viewing experience last longer.

For those with an interest in astronomy, the Christmas Star is a significant event because it allows comparisons to be made between Saturn and Jupiter in the same field of view.

Observations

One of the key observations was that before Dec. 20, Saturn was above Jupiter but after Dec. 20, Saturn was aligned with Jupiter. For astronomers, this illustrates that Saturn has a greater traveling speed than Jupiter relative to Earth’s orbit.

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This observation can further be measured as the distance between the planets decreased from 26 Jupiter widths to 10 Jupiter widths. It can also be inferred that the two planets have similar orbital shapes, relative to the earth. Interestingly, the two planets were tilted; Saturn has a tilt around 25 degrees while Jupiter only 3 degrees.

Clear picture

There were several challenges to obtaining a clear image of both planets.

A key challenge was to have clear details of both planets and the visibility of Jupiter’s moons. This is not easy to do as Jupiter and Saturn differ greatly in magnitudes. This was solved by taking different exposure lengths (10 seconds, 30 seconds), using a Barlow lens, and decreasing/increasing the gain/ISO.

Another challenge was the weather. After 7 p.m., the sky was often filled with clouds and it rained unexpectedly. To accommodate, my observation started as early as 6 p.m. and at higher elevations.

It was very exciting to watch the Christmas Star. It was an opportunity to learn the orbital positions of the two largest planets in the solar system. Likewise, this rare event gave me the opportunity to provide information to others about this event that occurs only once in several generations.

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