Moon is at its closest point to earth

The following sun and moon data for Dec. 11, 2008 is provided by the United States Naval Observatory.

Sunrise: 7:12 a.m.

Sunset: 4:51 p.m.

Moonrise: 3:46 p.m.

Moonset: 7:25 a.m. Dec. 12.

Moon phase: The moon is waxing gibbous with 99 percent of the visible disk illuminated. The moon is full Dec. 12 at 9:38 a.m. Mountain Standard Time.

The full moon dominates the night sky this week, and weather permitting, should put on a stunning show as it comes closest to Earth for 2008.

During the moon’s orbit around our planet, it travels at varying distances within a 31,000 mile range between it’s closest point to Earth — called perigee — and it farthest point from Earth — called apogee.

On May 19, 2008, the moon reached apogee and hovered at a spot 252,472 miles from Earth. Since then, the moon has gradually moved closer to Earth, and tomorrow will hover just 221,587 miles away — a difference of 30,885 miles.

It may seem difficult to imagine what a 30,000 mile difference might mean in astronomical terms, however astronomers say the moon will look 12 percent bigger and naked eye observers will clearly notice the difference.

In addition to being closer to Earth than any other full moon this year, the moon will also travel at a higher trajectory than normal. That said, stargazers who venture outside near midnight will find a brilliant full moon soaring almost directly overhead.

Each full moon has a variety of names, and the December moon is called either the “Moon Before Yule” or the “Long Nights Moon.” Both names are appropriate as tomorrow’s full moon is the last full moon before Christmas and December nights are some of the longest and darkest of the year.

Unfortunately, while the moon may provide a stunning spectacle in and of itself, it will also wash out much of the annual Geminid meteor shower which peaks between 10 p.m. and dawn Dec. 13. To compound the problem, the moon hovers in the constellation Gemini — the meteor shower’s radiant — through much of the night.

Nevertheless, and even with a fat, gibbous moon, stargazers based in dark sky locations such as Pagosa Country may glimpse as many as 10 Geminids per hour streaking across the night sky.

Mysterious Geminids

The Geminids are not ordinary meteors. While most meteor showers come from comets, Geminids come from an asteroid — a near-Earth object named 3200 Phaethon. But how does an asteroid make a meteor shower?

Comets do it by evaporating. For example, when a comet flies close to the sun, intense heat vaporizes the comet’s “dirty ice” resulting in high-speed jets of comet dust that spew into interplanetary space. When a speck of this comet dust (a meteoroid) hits Earth’s atmosphere traveling at 100,000 mph, it disintegrates in a bright flash of light — a meteor.

Asteroids, on the other hand, don’t normally spew dust into space and therein lies the mystery. Where did Phaethon’s meteoroids come from?

One possibility is a collision. Perhaps 3200 Phaethon bumped against another asteroid. Such a collision could have created a cloud of dust and rock that follows 3200 Phaethon around in its orbit. Such collisions, astronomers say however, are not very likely.

Another theory postulates that 3200 Phaethon used to be a comet, and astronomers point to a key bit of supporting evidence.

Phaethon’s orbit is highly elliptical, like the orbit of a typical comet, and brings the asteroid extremely close to the sun — twice as close as Mercury itself. Every 1.4 years, Phaethon swoops through the inner solar system where repeated blasts of solar heat could easily reduce a flamboyant comet to the rocky skeleton seen today.

If this scenario is correct, Phaethon-the-comet may have produced many rich streams of dust that spent hundreds or thousands of years drifting toward Earth until the first Geminid meteors appeared during the US Civil War. Since then, Geminids have been a regular shower peaking every year in mid-December.

3200 Phaethon is now catalogued as a “PHA” — a potentially hazardous asteroid — whose path misses Earth’s orbit by only 2 million miles. 3200 Phaethon measures 5 km wide, about half the size of the asteroid or comet that wiped out the dinosaurs 65 million years ago. It can be seen through backyard telescopes.

For those hoping to view the Geminids, observations are best begun after 8 p.m. while gazing toward the northeast.

Photo courtesy NASA
Dust from curious near-Earth asteroid 3200 Phaethon seems to fall from the constellation Gemini in this fisheye skyview. The composite image was recorded over four December nights (12-15) in 2006 from Ludanyhalaszi, Hungary. Of course, the streaks are meteor trails from the annual Geminid meteor shower. The work of astronomer Erno Berko, the finished picture combines 113 different frames and captures 123 separate meteors. The Geminid shower is one of the northern skies most reliably performing meteor showers and did not disappoint last year with rates of up to 100 meteors per hour. Unfortunately, moonlight will obscure all but the brightest Geminids this year and astronomers estimate skywatchers in dark sky locations may view up to 10 Geminids per hour.