Tonight – on December 29, 2016 – you can star-hop to the planet Neptune. Neptune, the eighth and farthest planet from the sun, is the only major planet that you absolutely can’t see without an optical aid. Even so, you can easily locate Neptune’s place in the sky, thanks to the two planets that you can see: Venus and Mars. There’s also a comet in the west after sunset; more below.
First look for dazzling Venus, the brightest star-like object in the evening sky, in the west at nightfall. Then seek out the much fainter yet clearly visible “star” above Venus. That’s the red planet Mars, which pales next to Venus but is still rather easy to see with the eye alone. An imaginary line from Venus through Mars lands right on Neptune.
Mars will have a very close conjunction with Neptune in just a few days. Depending on where you live worldwide, the twosome will appear closest together on the evening of December 31, 2016, or January 1, 2017. However, Mars and Neptune will be super-close together on both nights as seen from around the world. At a little over 1′ (1/60th of a degree) apart, these two worlds almost touch one another on the sky’s dome, easily taking stage inside a single binocular or telescopic field of view. For reference, 1′ is about 1/30th of the moon’s apparent diameter.
If you have an optical aid, and intend to search for Neptune, be sure to look soon as darkness falls, when all these planets are highest up for the night. Neptune will most likely look star-like and possibly blue in color. If you’re using binoculars, you may need to mount them to steady the view. A low-powered telescope may prove to be more advantageous for seeing this dim world. The real thrill is just to glimpse Neptune, no matter how modest its appearance. Unless the atmosphere is exceptionally steady and your’e using a magnification of 200X or so, you probably won’t discern Neptune as a disk.
Comet also in the west after sunset. Speaking of faint objects that require optical aid to be seen, there’s a comet in the sky now that observers with telescopes, telephoto lens and even binoculars are talking about. It’s Comet 45P/Honda-Mrkos-Pajdusakova, which, as expected, has brightened rapidly over the past couple of weeks and may now be visible to the unaided eye in a very dark sky.
The comet is not far from the planets shown on the chart at the top of this post – but even closer to the sunset – arcing across the sky in front of our constellation Capricornus, low in the sunset direction.
Bottom line: An imaginary line from Venus through Mars points to Neptune in late December 2016. Comet 45P/Honda-Mrkos-Pajdusakova is also in this part of the sky, but requires optical aid to be seen.