Bundle up and enjoy the Orionid meteor shower under moonless skies this week. Don’t miss the warm-up act, either, when two of Jupiter’s moons pair up in a captivating double-shadow transit.
This is such a great time of year to be out observing. I love looking up and tracing the band of the Milky Way from "summer" in Serpens across the top of the sky to "winter" in Auriga. It forms a bridge between opposite seasons uniting them in one grand simultaneity. But like many things that exist in a delicate balance this one won't last. Orion and its crew will see to that.
A sign of the coming change is the annual Orionid meteor shower. It peaks on the night of October 21–22 when we could see from 10 to 20 meteors per hour cracking off the Hunter's upraised club. The material originates with Halley's Comet. Approximately every 76 years at each return solar heating vaporizes some of the comet's dirty ice, releasing fluffy dust. Much falls back to coat the comet's surface but some of the particles escape to join the flotsam and jetsam distributed around Halley's orbit.
Earth crosses the comet's inbound leg beginning in late September and takes nearly two months to wade through it all — a journey of more than 147 million kilometers. We pass through the densest portion of cometary debris, better known as meteoroids, during the third week of October. When the particles strike the atmosphere, they heat up and incandesce. Halley's is the only major shower-producing comet whose path we intersect twice a year. May's Eta Aquariid shower originates from debris in the outbound leg of the comet's orbit.
Counterintuitively, ram pressure rather than friction is responsible for heating a meteor during its speedy plunge earthward. Ram pressure results when an incoming particle compresses — and therefore heats up — the air in its path. Temperatures on the incoming meteoroid's surface can reach more than 1650°C (3000°F). This all plays out between 80–120 kilometers (50–75 miles) altitude in the upper mesosphere and lower thermosphere, home to auroras and (at its lower limit) noctilucent clouds (NLCs). Heating also causes rapid ionization of meteoric and neighboring atmospheric molecules. Together the two processes are responsible for the bright flash of a meteor.
Orionids are swift. Striking the atmosphere at around 66 km/s (41 miles per second) they rapidly shed ionized iron, magnesium, sodium, and other elements. In time the ions latch on to water and other compounds and polymerize into larger nanometer-size smoke particles that can act as seeds for the formation of the NLCs. Comets get their fingers into everything, don't they?
The Orionids, a modest shower, radiate from a point near the Gemini-Orion border about 10° northeast of Betelgeuse. With the half-Moon setting between 11 p.m. and midnight local time on October 21st, conditions will be ideal for meteor-watching from midnight until dawn, October 22nd. Since the radiant culminates around 5 a.m. if I had to pick an ideal time slot, I'd set the alarm for 3 a.m. and spend an hour or two in a comfortable recliner under a big blanket with hot chocolate at the ready.
As with all showers you can face any direction you like since meteors will stream all over the sky. I like to face southeast with radiant off to the side for a happy mix of long and short-trailed meteors. It's also the direction of least light pollution from my location. The radiant is a vanishing point. The same way parallel railroad tracks appear to merge in the distance, meteors on parallel paths appear to stream from a point in the far distance in the direction of Orion. Just like those tracks, incoming meteoroids follow parallel paths but give the illusion of radiating in all directions from a single spot in the sky.
Orionids can be spotty. You might see two or three and then have to wait 10 or 15 minutes for the next. Watching a shower is a virtuous activity that requires patience. In the end it's less about the number of meteors observed as it is the experience and knowing that you needn't wait until 2061 to "see" Halley's Comet. Enjoy the show — the Orionids won't be moonless again until 2025.
As Jupiter approaches opposition on November 3rd, we're drawn to its brilliance. By 9 p.m. local time the gas giant gleams in the eastern sky not far from the sumptuous Pleiades star cluster. With the planet so big and bright it's fun to check it every clear night in the telescope to see what's new in the striped giant's dark belts and bright zones. Even small instruments will reveal the whimsical, night-to-night interplay of the four bright Galilean moons. Jupiter also routinely eclipses and occults the moons. They in turn cast tiny black shadows on the Jovian cloud tops during events called shadow transits. Single-shadow transits occur routinely, double plays are infrequent, and triple shadows are extraordinarily rare — the last occurred in January 2015 and the next will be on March 20, 2032.
On the night of October 19–20 Jupiter-watchers will witness an eye-catching double shadow transit of Ganymede and Io. This is a somewhat rare event during the current apparition because there are no double- shadow transits in November and just two in December — on December 23rd and 30th. The first is unfavorable for North America, while the other is primarily a twilight and early-evening event for the eastern half of the country. That's it till 2025!
The shadow pair will be in view on October 20th from 1:57 to 3:41 a.m. EDT (5:57 to 7:41 UT) and line up near the planet’s central meridian around 2:45 a.m. Io’s shadow will track along the South Equatorial Belt, while Ganymede, the biggest of Jupiter’s moons, will cast a considerably larger shadow over the South Polar Region. The choreographic details are below. Times are EDT and the data are from the RASC Observer's Handbook 2023. The bolded lines bracket the interval during which both shadows are visible simultaneously.
- Io's shadow begins transit of Jupiter 1:38 a.m.
- Ganymede's shadow begins transit 1:57 a.m.
- Io begins transit 2:01 a.m.
- Ganymede's shadow leaves Jupiter 3:41 a.m.
- Io's shadow leaves Jupiter 3:49 a.m.
- Ganymede begins transit 3:53 a.m.
- Io ends transit 4:10 a.m.
- Ganymede ends transit 4:42 a.m.
As with the Orionids the viewing time could be friendlier but October nights are long with plenty of time to catch up on sleep. Right? What, you don't believe me?