The moon turns full at 7:05 p.m. EST on Tuesday evening, Dec. 13, 2016. It looks pretty super tonight (Monday) as well.
What is a Supermoon?
The distance of the moon from Earth varies between approximately 362,600 and 405,400 km, because its orbit around Earth is elliptical, nor circular. A supermoon event occurs when the moon is closest to Earth (at perigee) and is lined up such that it is located behind Earth on the Sun - Earth line (a full moon) or is located between Earth and the Sun (a new moon).
The technical term for a supermoon is perigee-syzygy of the Earth-Moon-Sun system. In astronomy, the term syzygy refers to the straight-line configuration of three celestial bodies.
A full moon at perigee is visually larger up to 14% in diameter and shines 30% brighter than one at its farthest point, or apogee. Your mileage will vary — since weather conditions have a greater effect on the moon’s brightness than distance and the small variations in size are difficult to discern by the human eye for moons that are observed months apart.
Why is this Supermoon not as Super as the Nov 14 one?
Strictly speaking, a supermoon does not imply that the full (or new) moon occurs precisely at perigee; a supermoon is declared when the moon is within 90% of its closest approach to Earth in a given orbit. The supermoon of Nov 16 was 356,511 km away. The one this week will be 358,461 km away, not quite at perigee. Also, the perigee distance varies over time between 356,400 and 370,400 km due to variations in the moon’s orbit.
Hopefully, most of us cannot tell the difference.
How Often do Supermoons Occur?
Full moon supermoons occur approximately every 14th full moon. But they can occur multiple times in a row.
What About That Super-sized Moon at Moon-rise?
Low-hanging moons, can create what’s called a “moon illusion.” When the moon is near the horizon it can look unnaturally large when viewed through trees, buildings, or other foreground objects. The effect is an optical illusion, but that fact doesn’t take away from the experience.
This is one case, we are told, where we should not trust our lying eyes ;)
How Super was the Moon many many moons Ago?
The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact suggests that the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago.
At that time, the moon was about 20,000 - 30,000 km away from Earth, much closer than the distance today of about 400,000 km. The moon must certainly have looked super large - 13 to 20 times bigger - to whoever was around to observe it at that time.
The moon is still receding from Earth — at about 3 cm per year.
Supermoon Explained
Here is a video from NASA explaining some of the physics behind the super (sized) Moon.
The Geminid Meteor Shower 2016
Unfortunately, according to science.nasa.gov…, the supermoon this week will wipe out the view of the Geminid meteor shower. Bright moonlight will reduce the visibility of faint meteors five to ten fold, transforming the usually fantastic Geminids into an astronomical footnote.
Some suggestions on how to watch the Geminid meteor shower at earthsky.org/... and www.space.com/…
Height of the Moon in the Sky
This supermoon will traverse high in the sky in Northern latitudes, because it is winter time.
In Northern latitudes, during winter, the moon appears high in the sky, while in summer the moon appears low in the sky, opposite of how the Sun appears high or low with seasons.
Here is an explanation for the high/low elevation of the moon in summer/winter.
The orbital plane of the moon is close to the ecliptic, the orbital plane of Earth’s orbit around the Sun, inclined by 5.14 degrees. Unlike satellites of most other planets, our moon’s orbital plane is not close to Earth’s equatorial plane. Orbital period = 27.322 day. See en.wikipedia.org… for more details.
The orbit is not circular but slightly elliptical. Note that the perigee and apogee positions are always not aligned with the Earth-Sun line; the orbital plane is aligned with the star reference system. Hence, the perigee and the Earth-Sun line align approximately once a year.
The diagrams below show the orientation of the Earth with respect to the Sun in June and December.
In summer (June) for an observer in North America, we can see that the new moon lies between the Earth and the Sun and appears high in the sky (high elevation angle) at noon; the full moon (~14 days later) appears low in the sky at midnight.
In winter (December) for an observer in North America, we can see that the new moon appears low in the sky at noon; the full moon appears high (~14 days later) in the sky at midnight.
Why is the Moon’s Orbit Aligned with the Ecliptic and not the Equator?
en.wikipedia.org… states that for impact-generated moons of terrestrial planets not too far from their star (Sun), with a large planet–moon distance, it is expected that the orbital planes of moons will tend to be aligned with the planet's orbit around the star (the ecliptic) due to tides from the star, but if the planet–moon distance is small it may be inclined.
These conditions apply to our moon; hence, our Moon orbit is aligned with the ecliptic, rather than Earth’s equatorial plane.
Part of the reason is that the Sun's gravitational effect on the Moon is more than twice that of Earth's on the Moon. This effect obviously decreases with the outer planets and their satellites.
The Many Faces of the Moon
Concluding Remarks
I hope the weather cooperates and you get a chance to take some wonderful pictures on this last supermoon of 2016 tonight and tomorrow night. Please feel free to post them here.
The next supermoon dates are Dec 3, 2017 and Jan 1, 2018. Who knows if we will be around then; watching the dismal state of our world, the aliens might decide to simply blow Earth up, calling humanity a failed experiment and leave.
Notes
* Title photograph by ANTHONY AYIOMAMITIS, TWAN from news.nationalgeographic.com/…
Please visit my recent diary on Space Debris at www.dailykos.com/… and the one on Black Holes at www.dailykos.com/… for more science oriented stories.