I see the moon: Introducing our nearest neighbour

Mar 20, 2013 by Jonti Horner, The Conversation
The moon in total lunar eclipse as seen over Sydney in 2011. Credit: AAP/Sydney Observatory

The moon. Our nearest neighbour. The main source of the ocean's tides, and a beacon that drives the lives of animals across the globe. And also, to date, the only object beyond Earth on which humans have set foot.

Over the years, the moon has played a central role in the burgeoning understanding of our place in the universe. Yet despite its proximity to Earth, and the detail with which it has been studied, many of the moon's secrets still elude us.

In this, and another four articles, I'll introduce you to some of the ways in which the moon has influenced , how it has taught us about the formation of the solar system, and the past (and future) of our exploration of the solar system's strangest satellite. But first, some initial thoughts on our weird neighbour.

The moon: the stats

The moon orbits Earth at a mean distance of just over 384,000km. Its orbit is inclined to the plane of Earth's orbit around the sun by just over five degrees.

If we were looking at the Earth-moon system from beyond Earth's orbit, we would see the moon spends roughly half its time below the plane of Earth's orbit around the sun, and the other half above that plane.

I see the moon: Introducing our nearest neighbour
Neil Armstrong’s “small step for man” onto the moon was a defining moment of the 20th century. Credit: EPA/NASA

The location in its orbit at which the moon passes from below the plane of Earth's orbit to above that plane is known as the "ascending node". Over a period of around 18-and-a-half years, the location of the ascending node precesses around the moon's orbit completely.

Because the of Earth is tilted with respect to the plane of its orbit by 23-and-a-half degrees (the cause of the seasons), the moon's orbital tilt, with respect to the Earth's , varies over this 18-and-a-half year period between about 18 and 29 degrees.

The moon's orbit around Earth is also slightly eccentric – at perigee (its closest distance to Earth), it's a little over 362,500km from Earth, while at (furthest from Earth), it is almost 405,500km away.

The Earth-moon dynamic, explained. Credit: infringer1

That might not sound like much – around a 10% variation in distance – but it means that the apparent size of the moon in the sky can vary quite markedly, as can be seen below.

The moon is 1,737km in mean radius (compared to the Earth's 6,371km), making it the fifth largest satellite in the solar system. But even though it is ¼ the diameter of Earth, it is significantly less dense than our planet (at roughly 1/81 the mass of Earth).

The moon at its most extreme states of distance from Earth. Credit: Anthony Ayiomamitis/NASA

Even stranger is the fact that the near- and far-sides of the moon look so different. The near-side, familiar to anyone who has spent any time looking at the night sky, is dominated by the "mare", basalt outpourings that span a large fraction of surface.

The far side of the moon, by contrast, looks totally different – a dichotomy that has long puzzled researchers (although a recent study may have come up with an explanation).

The strangest planetary satellite?

In the four centuries that have passed since the discovery of the Galilean satellites (the four moons of Jupiter), the number of satellites known through our solar system has grown to more than 170 moons. And it's not just the planets that have been found to host companions: the solar system's smaller bodies are also regularly accompanied by their own satellites.

The dwarf planet Pluto is now known to have at least five satellites, and companions abound in every population of solar system object we study.

The size of Earth’s moon as compared to other moons in the solar system. Credit: NASA

While there are several satellites larger than our moon (Jupiter's Ganymede, Callisto and Io; and Saturn's Titan), those satellites are all dwarfed by their host planet. In stark contrast, when compared with Earth, the moon is huge.

The other large satellites in the solar system orbit their host planets almost perfectly in the plane of their equator. Yet our moon's orbit is inclined by between 18 degrees and 28 degrees to our equator.

So why is our moon so unusual? Why is it so different to so many of the other satellites in the ? The answer lies in the 's origin – and that's a story we'll return to shortly.

Explore further: Image: Siding Spring grazes Mars

add to favorites email to friend print save as pdf

Related Stories

How common are earth-moon planetary systems?

Sep 18, 2011

Sebastian Elser, Prof. Ben Moore and Dr. Joachim Stadel of the University of Zurich, Switzerland, in cooperation with Ryuji Morishima of NASA's Jet Propulsion Laboratory, tried to estimate how common Earth-Moon ...

Hinode witnesses solar eclipse

May 23, 2012

(Phys.org) -- Spectacular images from the Hinode spacecraft show the solar eclipse, which darkened the sky in parts of the Western United States and Southeast Asia yesterday.

How many moons does Earth have?

Jan 10, 2012

Look up in a clear night sky. How many moons do you see? Chances are, you’re only going to count to one. Admittedly, if you count any higher and you’re not alone, you may get some funny looks cast ...

Wobbly planets could reveal Earth-like moons

Dec 11, 2008

Moons outside our Solar System with the potential to support life have just become much easier to detect, thanks to research by an astronomer at University College London (UCL).

Image: Super Perigee Moon

Mar 21, 2011

(PhysOrg.com) -- The full moon is seen as it rises near the Lincoln Memorial, Saturday, March 19, 2011, in Washington.

Recommended for you

Image: Siding Spring grazes Mars

44 minutes ago

This excellent view of Mars seen together with Comet C/2013 A1 Siding Spring was captured by Scott Ferguson, Florida, USA, on 19 October 2014 on the morning that the enigmatic object made the closest-known ...

China to send orbiter to moon and back

3 hours ago

China will launch its latest lunar orbiter in the coming days, state media said Wednesday, in its first attempt to send a spacecraft around the moon and back to Earth.

NASA Webb's heart survives deep freeze test

13 hours ago

After 116 days of being subjected to extremely frigid temperatures like that in space, the heart of the James Webb Space Telescope, the Integrated Science Instrument Module (ISIM) and its sensitive instruments, ...

Cosmic rays threaten future deep-space astronaut missions

18 hours ago

Crewed missions to Mars remain an essential goal for NASA, but scientists are only now beginning to understand and characterize the radiation hazards that could make such ventures risky, concludes a new paper ...

MAVEN studies passing comet and its effects

20 hours ago

NASA's newest orbiter at Mars, MAVEN, took precautions to avoid harm from a dust-spewing comet that flew near Mars today and is studying the flyby's effects on the Red Planet's atmosphere.

User comments : 0