Get Ready for the Geminid Meteor Shower overnight this Friday/Saturday

As usual at this time of the year, the Earth is entering a stream of debris from rock comet 3200 Phaethon, which is the source of the annual Geminid meteor shower. Forecasters expect the shower to peak on Dec. 13-14 with as many as 120 meteors per hour.

This year the nearly full Moon will reduce the number of meteors you may see but it is still well worth a look. Expected to peak from about midnight Friday Australian Eastern Daylight Time (or 1300 UT) until 9pm (1000 UT) Saturday, this meteor shower will be visible in both hemispheres.

Though you do need to keep in mind that meteor showers often peak hours before or after predictions and for sure we certainly don’t know everything that a given meteor stream might have in store!

This shower is an interesting one though, with an equally interesting history and source. The Geminids were first identified as a distinct meteor shower by R.P. Greg of Manchester UK in 1862, and the estimated ZHR rose from about 20 to 80 through the 20th century. The parent source of this shower remained unknown until 1983, when astronomer Fred Whipple linked them to the strange “rock-comet” body 3200 Phaethon. This is an Apollo asteroid also thought to be a member of the Pallas family of asteroids, 3200 Phaethon seems to be shedding enough material to produce the annual Geminid meteor shower. This makes the annual shower rare as one not produced by a comet. It’s worth noting that 3200 Phaethon also passes extremely close – 0.14 AU – from the Sun at perihelion, and gets periodically “baked” during each 1.4 year passage.

In the 21st century, rates for the Geminids have stayed above a Zenith hourly rate (ZHR) of 120, now the highest of any annual shower. It’s worth noting that an extrapolated ZHR of almost 200 were seen in 2011 when the Moon was at an equally unfavorable waning gibbous phase! The Geminids always produce lots of fireballs, capable of being seen even under moonlit skies.

With our warmer nights down under it is a great time to get out and have a look! Jupiter is also looking good after about 10:30pm and Mars and Saturn are visible in the early dawn skies as well.

“SuperMoon” this Sunday

The biggest and brightest full moon of the year arrives tomorrow as our largest natural or otherwise satellite comes a little closer than normal. It will, at least from our perspective on Earth appear a bit bigger – a good experiment – take a photo tonight and take one next full moon in the same place and see if it is true.

The term ‘Supermoon’ is a nickname for a perigee full moon, this is the when the Moon is  closer to the Earth than usual in its orbit. Apogee and perigee refer to the distance from the Earth to the moon. Apogee is the furthest point from the earth while Perigee is the closest point to the earth and it is in this stage that the moon appears larger. Looking at the moon in the sky without anything to compare it to, you wouldn’t notice any size difference. But the difference in size can in fact be quite significant.

full moon at apogee and perigee

If you were to take a picture when the Moon is at perigee and again at apogee using the same camera and lens you would notice the difference.

The full Moon occurs at 1:35pm (AEST) Sunday May 6th in Australia. It is predicted that the moon will about 14 per cent brighter than usual.

Sunday’s event is a “supermoon,” the closest and the biggest and brightest full moon of the year. At 1.35 p.m., the moon will be about 356,956 km from Earth. That’s about 24,653 km closer than it is on average.

That proximity will make the moon appear about 14 percent bigger than it would if the moon were at its farthest distance, however, the difference in appearance is so small that you will find it hard pick it with your unaided eye.

The moon’s distance from Earth varies because it follows an elliptical orbit not a circular one.

Like any full moon, tomorrow’s moon will look bigger when it’s on or near the horizon rather than higher in the sky, thanks to an optical illusion. The full moon appears on the horizon at sunset. On the East coast, for example, that will be a at 5.07pm.

The last “supermoon” on March 20, last year was about 380 km closer than this year’s will be. Next year’s will be even a bit farther away than this year’s. Each year there is a perigee and an apogee Moon and the distances vary.

One effect that can be noticed doesn’t affect me where I live, but coastal folks are very familiar with the tides and how their height varies over the course of a month, again, due to the Moon not always being the same distance from the Earth. As the Moon’s orbit brings it in closer proximity to our planet, its gravitational forces can increase by almost 50%, and this stronger force leads to high tides. Likewise, when the Moon is farther away from the Earth the tides are far less spectacular.

The Moon’s influence can also be balanced out by the position of the Sun – if the Sun and the Moon find themselves 90 degrees apart in relation to an observer on the Earth, then high tides are not as high as they normally would be. This is because despite its greater distance from the planet, the Sun’s mass allows it to exert enough gravitational force on the oceans that it can negate some of the effects of the Moon’s pull. This phenomenon of lower high tides is called a neap tide. In the same way, when the Sun lines up with the Moon and the Earth, as during a Full Moon, then the Sun can act to amplify the tidal forces, drawing even higher tides. These are known as spring tides, named not for the season, but for the fact that the water “springs” higher than normal. The variance in the height of the world’s tides also depends on the local geography of the coastline and the topography of the ocean floor.

Planetary Alignments in the Evening Sky

Over the next 4 weeks, the solar system’s brightest planets will be putting on a spectacular evening show as they start to move into formation over the nights to come.

If you go out just after sunset and look towards the west, you will see Venus and Jupiter popping out of the twilight even before the sky has gone completely dark. After you have found them once or twice you will be able to find them earlier.   Seeing these two brilliant planets surrounded by darkening blue of the evening sky is a lovely sight.

If you go out at the next night, the view improves, because Venus and Jupiter are converging.  In mid-February they were about 20 degrees apart but by the end of the month, the angle narrows to only 10 degrees—so close that you can hide them together behind your outstretched palm.  Their combined beauty grows each night as the distance between them shrinks.

 A special night to look is Saturday, February 25th, when the crescent Moon moves in to form a slender heavenly triangle with Venus, Jupiter and the Moon as its vertices.  One night later, on Sunday, February 26th, it happens again.  This arrangement will be visible all around the world, from city and countryside alike.  The Moon, Venus and Jupiter are the brightest objects in the night sky; together they can shine through city lights, fog, and even some clouds.

 After hopping from Venus to Jupiter in late February, the Moon exits stage left, but the show is far from over.

 In March, Venus and Jupiter continue their relentless convergence until, on March 12th and 13th, the duo lie only three degrees apart—a spectacular double beacon in the sunset sky. Now you’ll be able to hide them together behind a pair of outstretched fingertips.

 There’s something mesmerizing about stars and planets bunched together in this way. This strange phenomenon is due to the fact that your eye works in much the same way as a digital camera does. In front, there is a lens which focuses the light and the retina acts like a photo-array behind the lens to capture the image of what you see. The retina is made up of rods and cones which are the organic equivalent of electronic pixels.

 There’s a tiny patch of tissue near the centre of the retina where cones are extra-densely packed. This is called “the fovea.” This enables you to see objects in high definition – it is critical to everyday tasks such as reading, driving and watching television. The fovea has the brain’s attention.

 The field of view of the fovea is only about five degrees wide. Most nights in March, Venus and Jupiter will fit within that narrow cone.  And when they do—presto!  It’s spellbinding astronomy.