This Week: Neptune Opposition 2018 and Comet 21P!

Neptune image by Voyager 2 probe. NASA/JPL

With the moon a waning crescent, it is a good time to pick out some faint objects in the sky.

On Friday, September 7th, Neptune rises opposite the sun and is closest to earth. Being in the sky all-night, and at it’s brightest, presents a good opportunity to sight this rarely-seen telescopic planet. Unlike the naked eye planets, you’ll have to crank-up the power to make sure you’ve really found it. Even at 100x it just looks like a magnitude 7.8 star. So don’t expect to see a Voyager 2 quality image through your eyepiece.

Click on image to enlarge. Neptune (dots showing position from Sept. 5 to 12) is just south of a line between Phi- and Lambda-Aquarii. In early September it will be moving slowly toward the Lambda star. Using your finder, or a wide-field eyepiece, setup your field on the stars labeled by magnitude inside the 5° circle. Over the course of the week, Neptune will be approaching a line drawn between the mag. 7.5 and 7.4 stars shown. Neptune will be slightly dimmer than both at mag. 7.8.  83-Aquarii it is the brightest of the stars labeled by magnitude — shining at  mag. 5.5 — and will help you orient the chart.

The field containing Neptune is on a line between  Phi-Aquarii (mag. 4.2) and Lambda-Aquarii (mag. 3.7). At opposition, the planet will be less than a degree north of 83-Aquarii (mag. 5.5). Use these three stars marked in the chart above to initially frame your field and then refer to the stars with labeled  magnitudes to pick out the planet.

Nagler 3-6 Planetary Zoom.

With the Nagler 3-6mm Planetary Zoom you won’t have to switch eyepieces to ferret out Neptune from the field stars. Just find the field containing the planet, center on your best suspect, and twist the black barrel to zoom in. If you hit upon it, you’ll be rewarded with the sight of a planetary disk. Depending on scope aperture, you can see the blue hue of the planet. With an 8″ or larger scope you can try for Triton (mag. 13.5) the major moon of the Neptunian system. Use the S&T Triton Tracker to help locate that.

The discovery of Neptune is a story of missed opportunity. Galileo observed Neptune in December of 1612 and there is evidenced that he noted that it changed position relative to the fixed stars. He didn’t pursue it further though. Centuries later, Cambridge Observatory director, James Challis, unknowingly spotted the planet twice, in August 1846, based on other astronomer’s predicted location of a planet that was perturbing the orbit of Uranus. But he lacked the proper sky charts to identify it as a planet. On September 23, 1846 astronomer Johann Galle (Berlin Observatory) used a location prediction by French mathematician Urbain Le Verrier to confirm Neptune’s existence.  

Comet 21P/Giacobini-Zinner in the Morning Sky
Giacobini-Zinner swings through the inner solar system every 6.6-years. This week, it is making a run in the morning sky through the heart of Auriga (“The Charioteer”). It reaches perihelion (closest) with the Sun on September 10th at less than 0.4 AU from Earth. The comet is not expected to become naked-eye, instead content to glow around mag. 7 in our skies.

Click to enlarge. Chart shows daily positions of Comet 21P at 0-UT from Sept 5 – 12. It’ll be highest in the sky before sunrise so this is for early risers or night-owls. You’ll have to “sweep” around the heart of Auriga to find the comet.
Tele Vue 24mm Panoptic will give you the widest true field in 1¼” eyepieces.

The comet is moving quickly and the best way to spot it is to point the scope in the general area shown in the chart above and then move the scope to “sweep up” the comet using an eyepiece that can cover a wide field.  For 1¼” eyepiece holders that would be our 24-mm Panoptic or 32-mm Plössl eyepieces. For 2″ eyepieces we have many choices. Any of the longest focal lengths in the Panoptic, Plössl, Nagler, or Ethos line can do the job. You just have to avoid large exit pupils in obstructed scopes. Our Eyepiece Calculator accounts for this and is a handy tool for finding just the right eyepiece for your scope. 

Initially reported by Michel Giacobini in 1900 and  confirmed by Ernst Zinner upon return in  1913,  the “P” in “21P” designates it as a “periodic” comet and the “21” is just the order that it was put on the periodic list comets. This growing list has about 370 entries so far.

Update 2018 Sept 10
Click image to see animation.

Update 2018 Sept 07
Click image to see comet.

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