What’s in the Sky in December
Image: Geminids 13th December, 02:00 hrs AEDT Local Time.
December
- 6th Earliest Sunrise for the year.
- 8th Mars is rising 05:18hrs AEDT in Ophiuchus, Mag. +1.4 Dist. 366.9 milion km, 20.40 light min.. Tilt of North pole towards Earth is 14.0413 degrees, 97.78% illuminated.
- 17th Moon at Apogee 05:42hrs AEDT, Dist. 404,347.0km.
- 8th Saturn at mag. 1.0 setting at 00:26hrs AEDT, Dist. 1,486.7 million km.
- 8th Jupiter is a beautiful evening object setting at 04:02hrs AEDT, mag. -2.8, dist. 6.520 million km, Red spot transits Fri Dec 8th 19:19hrsAEDT.
- 9th Red Spot on Jupiter transit 05:15hrs AEDT.
- 9th One of the GREAT COMETS, Comet 1P/Halley, reaches it’s furthest point from the Sun on the 9th December 2023 and begins the journey back to reach perihelion on Saturday September 17th, 2061.
- 8th Mercury sets at 22:46hrs AEDt, Dist. 139.6 million km Mag. -0.3.
- 4th-17th Geminids Meteor Shower Maximum: December 14, 19h UT (15th 06hrs AEDT); ZHR = 150; Velocity = 35 km/s.
The best and most reliable of the major annual showers presently observable reaches its broad moon-free maximum on December 14 centred at 19h UT. Well north of the equator, the radiant rises about sunset, reaching a usable elevation from the local evening hours onwards. In the southern hemisphere, the radiant appears only around local midnight or so. It culminates near 02h local time. During high activity, observers
should report their count data for short intervals (no longer than 15 minutes).
Remember that there may be activity related to comet 46P/Wirtanen* on December 12, perhaps from two different radiants. While counting is the best method to record the major GEM activity, notes are advised about the apparent trails and angular velocities of possible “Wirtanen-meteors”.
*12th December ~ 22:20hrs AEDT – Possible Meteor shower
Just before the Geminid maximum there is a chance to observe meteors released from comet
46P/Wirtanen (the initial target of the Rosetta comet exploring mission) on December 12,
around 11h20mUT (λ = 260 .◦11). Vaubaillon writes: this is the first time I see a trail of this
comet crossing the Earth’s orbit. The 1974 trail is quite young and we have no clue concerning
the flux density. Most interestingly, the radiant is split in two very different regions in the sky
– something requiring detailed data and analysis. Most of the activity should be related to a
radiant at about 10◦ north of α Phoenicis in Sculptor, the other radiant is
at between α Pegasi and γ Piscium. Meteors should be very slow with
V∞ = 10km/s and V∞ = 13km/s, respectively. The described activity is expected shortly before
the Geminid maximum. While the Geminid meteors are preferably counted, we recommend to
note to which of the two radiants potential meteors are associated (Scl or Psc). Of course, video
data will provide complete data sets to investigate the radiant structure. Source: IMO
- 17th Moon at Perigee 05:55hrs AEDT Perigee distance 367,901.4km.
- 22nd South Solstice (or Summer Solstice in Southern Hemisphere) 13:28 AEDT. The Sun is at it’s Southern most position, -23 26′ 15.07″
- 27th Moon lowest 28 degrees Nth and is at full Moon.
- MOON’s 18.6 year cycle
How the Moon Devastated a Mangrove Forest
In 2015 the moon’s wobble and an El Niño teamed up to kill off tens of millions of Australian mangroves
The mystery emerged in 2015, when nearly 10 percent of the seemingly healthy mangrove forest along northern Australia’s Gulf of Carpentaria suddenly died. Scientists initially blamed this crucial ecosystem’s die-off solely on an unusually strong El Niño, a weather pattern that periodically siphons water away from the western Pacific and lowers local tides. But a new study published in Science Advances reveals that El Niño had a stealthy accomplice: the moon.
Researchers analyzed more than 30 years of national satellite data to narrow down the suspect list. “It was just the most phenomenal data set,” says the study’s lead author Neil Saintilan, a biogeographer at Macquarie University in Australia. A pattern quickly emerged—about every 18 to 19 years, mangrove tree cover along the Gulf of Carpentaria thinned out significantly before bouncing back to normal within a couple of years. And roughly nine years after each such die-off, the mangrove canopies became unusually dense.
That regularity gave the researchers an important clue. “Nature’s usually pretty chaotic,” Saintilan says. “If something is superregular, it’s probably some kind of orbital cycle.”
“The 18.6-year cycle is essentially driven by what we call a ‘wobble’” in the moon’s orbit, says climate dynamics researcher Sophie Wilmes, who studies tides at Bangor University in Wales and was not involved in the mangrove investigation. Lunar gravity affects daily ocean tides worldwide. As the moon’s orbit oscillates, or wobbles, over 18.6 years, it creates regular, sustained periods of unusually high or low tides in certain places. This effect is especially strong in the Gulf of Carpentaria; its low tides can drop by an average of 40 centimeters because of its location relative to the equator and the shape of Australia’s coastline.
Indeed, the researchers found that Australia’s 2015 mangrove die-off fell 18 and a half years after the previous one. And because of the 2015 El Niño, the trees were hit with a low-tide double whammy: El Niño decreased tides an additional 40 centimeters, a fatal blow to water-loving mangroves.
Although the orbital mechanics of the moon’s wobble have been studied extensively, “there hasn’t been much work that looks at its impact on ecology,” Wilmes says, “so it’s a really cool paper.” In the future, Saintilan and his colleagues hope to look at whether this phenomenon affects mangrove forests in other parts of the world. They also want to study how sea-level rise driven by climate change will alter this natural ecological pattern. A moderate rise might mitigate some of the tidal drop, helping to preserve mangrove forests, but an extreme rise could drown the trees at the cycle’s highest tidal point. “We might be able to anticipate when—or if—we’ll start to see some big problems in terms of mangroves coping,” Saintilan says.
