Today Mercury will be at inferior conjunction. After today the planet will reappear in the dawn, rising high enough from the Sun’s glow to be seen around the end of the year.
Inferior conjunction is when the planet passes between the Sun and the Earth. As such the only planets to see inferior conjunction are Mercury and Venus. A transit is possible if the planet passes directly in front of the Sun, but normally this alignment does not occur, the planet passing above or below the Sun as seen from the Earth. There are no transits of Mercury in 2017, the next will be Nov 11, 2019.
The annual Geminid meteor shower has become one of the most reliable annual meteor showers. Known for bright and slow moving fireballs the Geminids can provide a good reason to spend a few hours outside on a December night. This shower has routinely provided rates above 100 meteors per hours in past years, this year should be no exception.
First observed over 150 years ago this is a interesting meteor shower. The parent body for the Geminids is not a comet as with most showers, but rather the asteroid 3200 Phaethon. It is somewhat of a mystery how this mostly rocky body gives rise to the debris stream needed to generate a meteor shower. The asteroid does orbit well inside the orbit of Mercury every four years, where intense solar heating may heat trapped ice and liberate loose material from the surface.
The peak is expected to occur around December 14, 06:30UT. For viewers here in Hawaiʻi this occurs on the evening of December 13th. The Geminids feature a broad peak, with high rates for nearly 24 hours, thus allowing the all time zones a decent chance to enjoy the show.
There should be no substantial moonlight to drown out this years showing, dark skies to allow even the faintest meteors to be seen. The Moon is a thin waxing crescent, 1% illuminated on the morning of the 13th, essentially new.
The Geminid radiant rises round 8pm, thus meteors should be visible all night long. You can stay up late or set the alarm early, your preference. Southern viewers will have to wait until around midnight for the constellation Gemini to rise, making this a morning shower.
Watching meteors requires no more equipment than your eyes and a dark sky, and can be enjoyable for just about anyone. Well? It is December, you should probably add a warm coat to the equipment list.
Full Moon will occur today at 05:47HST.
This full Moon will be a perigee full Moon, with the Moon appearing a bit larger and brighter. Today lunar perigee occurs about 17 hours of full, producing a full moon that is about 12% larger and 30% brighter than if full occurs while the Moon is at apogee.
The forecast looks promising. Not for observing, we lost most of the night last night to fog and clouds, the rest of the week looks worse. It does look like we may get some snow this week. Checking the cameras there are already a few flakes coming down, just a degree or so too warm to stick.
Winter is here… Snow for Christmas?
Update! Sticking now…
Today Mercury will be at maximum eastern elongation, as high in the evening sky as it will appear for this current apparition. After today the planet will slide back into the sunset, passing through inferior conjunction on December 12th to reappear in the morning sky around the end of the year.
Mercury typically completes three morning and three evening apparitions in each year. While the innermost planet never gets very far from the Sun, maximum elongation represents the best time to observe Mercury as high in the sky as possible.
There are no transits of Mercury in 2017, the next will be Nov 11, 2019.
New Moon will occur today at 01:42HST.