What's Up, by Jared May: September 16 - 22

What’s Up This Third Week of September

The forecast is showing the possibility of partially clear skies near the middle and end of this week. Hopefully, those turn into cloudless nights as the weather models get updated. The temperature drops into the low-70s and upper-60s in the few hours following sunset, which occurs around 7:30 PM this week. Be on the lookout for a razor-thin crescent moon, Neptune at opposition, early-morning Mercury, and the Helix Nebula.

The new moon was the past Friday, September 15th, so expect the waxing crescent to grow brighter and brighter over the next week. Look low in the western sky shortly after sunset these next few days to spot the thin crescent moon with a chance to see some earthshine. This is an ideal time to view the moon through a telescope or binoculars since it is shining at magnitude -5.5 (as opposed to a full moon which shines at magnitude -12.6, or over 100x brighter) and won’t hurt your eyes.

The moon on the evening of September 16 (simulated)

On Tuesday, September 19, Neptune will reach opposition, or in other words, be in the opposite location of the sun. This outer planet which currently shines at magnitude 7.8 (combined with its small apparent size) requires a decently powerful telescope and dark skies to view it. Once you have pointed your telescope in the correct direction, you’ll want to look for a noticeably blue-colored star – this is Neptune. In 1999, the Bridge Creek Oklahoma tornado had one of the fastest windspeeds recorded on Earth at 302mph. Neptune crushes this record with windspeeds hitting 1,200mph in its methane atmosphere! [Image: https://www.nasa.gov/image-feature/discovering-neptune]

Neptune from the only spacecraft that ever paid it a visit: Voyager 2.

Neptune is challenging to spot, and too faint to be seen without optical aid, but it’s striking blue color makes it a worthwhile object to hunt down in a small telescope.

The innermost planet, Mercury, will reach its greatest western elongation this Friday, September 22nd. Greatest western elongation occurs when a planet appears to be at the “top” of its orbital path extending westward from the eastern horizon. Since Mercury’s orbit is inclined by 7 degrees relative to Earth’s, Mercury’s mostly-circular orbit appears highly elliptical. To see this event, look low in the eastern sky between 6 AM and 7 AM to spot Mercury leading the sun and extending “high” in the sky.

Mercury will be a sight for early risers when it reaches its greatest western elongation.

Any time this week, look in the southern sky with some binoculars about 10° below Saturn and 5° to the right. In this region of the sky, you’ll find NGC 7293, more commonly known as the Helix Nebula. This deep-sky object emits its vibrant colors (unfortunately only visible in a camera, not our eyes) at magnitude 7.6, so dark skies will certainly help. It is a relatively large object, spanning about half a degree across, or the same as a full moon. Detailed images have revealed a single star at the “pupil” of this eye-shaped nebula. This single star has been shedding its outer layers producing this beautiful cosmic display as part of the dying stages of stellar evolution. [image: https://esahubble.org/images/opo0432b/]

The helix nebula, one of the brightest and closest of the planetary nebulae, seen in this deep image from the Hubble Space Telescope.

Get outside and enjoy the potentially clear skies throughout the middle and end of this week. Make sure to bring a blanket and a jacket to stay warm – the cooler temperatures are hinting at the upcoming autumn season. Kickback and enjoy the beauty of the cosmos overhead. Try spotting the waxing crescent moon, distant Neptune with ripping winds, Mercury at the “top” of its orbital hill, and a bright and vibrant Helix Nebula from a dying star.

Clear Skies!