We will be visited by a bright green comet this December just before Christmas! The comet’s official name is Comet 46P/Wirtanen however because of the timing I think calling it the Christmas Comet (2018) sounds a lot better.
The Christmas Comet is currently the closest thing to the Earth outside of the moon and will be getting a lot closer. On December 16th 2018 the Christmas Comet will make its closest approach at .0775 AU (That’s 7.7% the earth sun distance, 7.2 Million Miles, 39 light seconds, or 30 times the earth moon distance). It will be passing in the sky right between the Pleiades (the seven sisters) and Aldebaran (the red eye of Taurus the bull) when it makes closest approach on the 16th.
sky and telescope magazine has a good image of where the comet will be in the sky for the next few months here: https://s22380.pcdn.co/wp-content/uploads/Comet_46P_Dec18-Jan19.jpg
The Christmas Comet should be fairly visible reaching about the size of the full moon. A decent pair of binoculars will probably offer the best views as a telescope might zoom too much and it might be too dim to see easily with the unaided eye. It is hard to predict brightness of comets but the Smithsonian is saying about 8+ magnitude (about the brightness of Neptune, would require a small telescope or good binoculars to see), and sky and telescope magazine is more optimistic saying it should be about 3+ magnitude (just dimmer than the north star, would be able to see with the unaided eye).
The main factor working against the comet will be the moon right now it is a new moon meaning in the coming weeks the moon will be growing causing more and more natural light pollution and setting later and later. On the night of the 16th the moon will be 56% full and setting just after midnight making it highly likely to wash out the comet’s appearance to many viewers. On Christmas eve the moon will rise only 45 minuets after sunset giving a moonless evening to view the Christmas comet.
The word comet comes from Latin and means “long haired star” referring to the hair like tail, the most striking feature of a comet. The Christmas comet should appear with a faint cloud tail possibly with a green hue pointing away from the sun.
Comets are referred to as dirty snowballs as they are primarily made up of ice with some rock and dust mixed in. The tail of a comet always points away from the sun because the heat of the sun and the solar wind (a stream of charged particles the sun radiates) hits the comet with enough power to melt the ice and burn the dust. Watching the Christmas Comet between the 13th and the 19th the change in direction of the tail should be very apparent, flipping from pointing south to pointing north as the comet crosses the ecliptic.
Up to the 1700s Saturn was seen as the limit of our solar system. Comets of course were known of prior to the 1700s as they are quite striking when they appear, however comets were seen as foretelling a large event. So instead of studying them when they appear many focused mainly on what upcoming event is being foreshadowed. Aristotle and many others thought comets were phenomenon which occur in the Earth's atmosphere. Edmund Halley famously predicted a comet he had seen in 1682 to return in 1758 this was the first insight into what comets really were. Halley’s comet meant that the solar system was much larger than previously thought as, the orbit Halley predicted had his comet traveling just outside of the orbit of Neptune (which wasn’t discovered until 1846) meaning the comet traveled over 3.7 times as far as Saturn’s orbit. The Christmas comet's orbit is much smaller never traveling further away from the sun than Jupiter's orbit.
If you wish to see the Christmas comet up close in one of my large telescopes and learn more about it I will be offering 2 special tours for the comet, one on December 14th and one on December 16th. Contact me soon for booking.