What’s in the Sky in December
December
- 10th Moon lowest 27degrees Nth.
- 12th Moon at Apogee 11:30hrs AEDT, Dist. 405,870.4km.
- 18th Mars is rising 19:43hrs AEDT in Taurus E of Aldebaran, a very bright object, Mag. -1.6 Dist. 85,670,222km. Tilt of North pole towards Earth is -7.059181 degrees.
- 18th Saturn at mag. 0.8 setting at 00:06 AEDT on the 5th, Dist. 1,550, 868,476km. Tilt of Saturn’s North pole towards Earth is 14.1954 degrees.
- 18th Jupiter is a beautiful evening object setting at 01:49hrs AEDT, mag. -2.5, dist. 716,017,486km, tilt of North Pole towards Earth is 2.1643 degrees.
- 18th Mercury sets at 22:14hrs AEDT, Dist. 163, 084, 137km Mag. -0.6.
- 4th-17th Geminids Meteor Shower Maximum: December 14, 07h UT ( 18Hr AEDT); ZHR = 150; Velocity = 35 km/s. The best and most reliable of the major annual showers presently observable reaches its broad maximum on December 14 centred at 07h 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. Even from more southerly sites, this is a splendid stream of often bright, medium-speed meteors, a rewarding event
for all observers, whatever method they employ.
The peak has shown little variability in its timing in recent years, with the more reliably-reported maxima during the past two decades all having occurred around 2021 December 13, 14h to December 14, 12h UT. The peak ZHRs have shown a slight increase over a longer period and reached 140{150 in all recent years. Usually, near-peak Geminid rates persist for several hours, so much of the world has the chance to enjoy something of the shower’s best. Mass-sorting within the stream means fainter meteors should be most abundant almost a day ahead of the visual maximum. The 2021 return occurs only four days before full Moon. Depending on the latitude, the moon sets around 02h local time and leaves about 3{5 hours for observations in a dark sky. Source: IMO
- 22nd South Solstice (or Summer Solstice in Southern Hemisphere) 03:00AEDT. The Sun is at it’s Southern most position, -23 26′ 07.39″
- 24th Moon at Perigee 19:30hrs AEDT Perigee distance 358,270.4km, Moon is furthest South 27 Degrees.
- 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.
