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The Moon in Motion

The Moon in Motion

Happy New Year! And happy supermoon! Tonight, the Moon will appear extra big and bright to welcome us into 2018 – about 6% bigger and 14% brighter than the average full Moon. And how do we know that? Well, each fall, our science visualizer Ernie Wright uses data from the Lunar Reconnaissance Orbiter (LRO) to render over a quarter of a million images of the Moon. He combines these images into an interactive visualization, Moon Phase and Libration, which depicts the Moon at every day and hour for the coming year. 


Want to see what the Moon will look like on your birthday this year? Just put in the date, and even the hour (in Universal Time) you were born to see your birthday Moon.

Our Moon is quite dynamic. In addition to Moon phases, our Moon
appears to get bigger and smaller throughout the year, and it wobbles! Or at
least it looks that way to us on Earth. This wobbling is called libration, from
the Latin for ‘balance scale’ (libra). Wright relies on LRO maps of the Moon
and NASA orbit calculations to create the most accurate depiction of the 6 ways
our Moon moves from our perspective.

1. Phases


The Moon phases we see on Earth are caused by the
changing positions of the Earth and Moon relative to the Sun. The Sun always
illuminates half of the Moon, but we see changing shapes as the Moon revolves
around the Earth. Wright uses a software library called SPICE to calculate
the position and orientation of the Moon and Earth at every moment of the year. With his
visualization, you can input any day and time of the year and see what the Moon
will look like!

2. Shape of the Moon


Check out that crater detail! The Moon is not a smooth sphere.
It’s covered in mountains and valleys and thanks to LRO, we know the shape of
the Moon better than any other celestial body in the universe. To get the most
accurate depiction possible of where the sunlight falls on the lunar surface
throughout the month, Wright uses the same graphics software used by Hollywood
design studios, including Pixar, and a method called ‘raytracing’ to calculate
the intricate patterns of light and shadow on the Moon’s surface, and he checks
the accuracy of his renders against photographs of the Moon he takes through
his own telescope.


3. Apparent Size 


Moon Phase and Libration visualization shows you the apparent size of the Moon.
The Moon’s orbit is elliptical, instead of circular – so sometimes it is closer
to the Earth and sometimes it is farther. You’ve probably heard the term
“supermoon.” This describes a full Moon at or near perigee (the point when the Moon is
closest to the Earth in its orbit). A supermoon can appear up to 14% bigger and brighter
than a full Moon at apogee (the point when the Moon is farthest from the Earth
in its orbit). 

Our supermoon tonight is a full Moon very close to perigee, and will appear to be about 14% bigger than the July 27 full Moon, the smallest full Moon of
2018, occurring at apogee. Input those dates into the Moon Phase and Libration visualization to see this difference in apparent size!

4. East-West Libration

Over a month, the Moon appears to nod, twist, and roll. The
east-west motion, called ‘libration in longitude’, is another effect of the
Moon’s elliptical orbital path. As the Moon travels around the Earth, it goes
faster or slower, depending on how close it is to the Earth. When the Moon gets
close to the Earth, it speeds up thanks to an additional pull from Earth’s gravity. Then it
slows down, when it’s farther from the Earth. While this speed in orbital
motion changes, the rotational speed of the Moon stays constant. 

This means
that when the Moon moves faster around the Earth, the Moon itself doesn’t rotate
quite enough to keep the same exact side facing us and we get to see a little
more of the eastern side of the Moon. When the Moon moves more slowly around
the Earth, its rotation gets a little ahead, and we see a bit more of its
western side.

5. North-South Libration


Moon also appears to nod, as if it were saying “yes,” a motion called
‘libration in latitude’. This is caused by the 5 degree tilt of the Moon’s
orbit around the Earth. Sometimes the Moon is above the Earth’s northern
hemisphere and sometimes it’s below the Earth’s southern hemisphere, and this
lets us occasionally see slightly more of the northern or southern hemispheres
of the Moon! 

6. Axis Angle


Finally, the Moon appears to tilt back and forth like a metronome.
The tilt of the Moon’s orbit contributes to this, but it’s mostly because of
the 23.5 degree tilt of our own observing platform, the Earth. Imagine standing
sideways on a ramp. Look left, and the ramp slopes up. Look right and the ramp
slopes down. 

Now look in front of you. The horizon will look higher on the
right, lower on the left (try this by tilting your head left). But if you turn
around, the horizon appears to tilt the opposite way (tilt your head to the
right). The tilted platform of the Earth works the same way as we watch the
Moon. Every two weeks we have to look in the opposite direction to see the
Moon, and the ground beneath our feet is then tilted the opposite way as well.

So put this all together, and you get this:

Beautiful isn’t it? See if you can notice these phenomena when you observe the Moon. And keep coming back all year to check on the Moon’s changing appearance and help plan your observing sessions.

@NASAMoon on Twitter to keep up with the latest lunar updates. 

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