Australasian Science: Australia's authority on science since 1938

The Earth Had Two Moons

By Dave Rebeke

There’s evidence that the Earth once had two moons, while light pollution obscures the Milky Way from one-third of the population.

The Earth had two moons until one of them smacked into its big sister in what scientists from the University of California, Santa Cruz are calling “the big splat”. As a result, our planet is now overlooked by a single bulked-up and slightly lopsided Moon.

This all supposedly happened about 4.4 billion years ago, long before there was any life on Earth to appreciate the view.

The young moons had formed about 100 million years earlier when a giant proto-planet smashed into our planet. They both orbited Earth and rose in the sky together, the smaller one trailing a few steps behind like a little sister in tow.

The new model suggests that the lunar far side highlands could have been created from such a collision, and goes a long way to explaining why the near and far sides of the Moon are so puzzlingly different. The near side is relatively low and flat, while the topography of the far side is high and mountainous, with a much thicker crust.

These new studies build upon the “giant impact” model for the origin of the Moon, in which a Mars-sized object collided with Earth early in the history of the solar system and ejected debris that coalesced to form the Moon.

Advanced computer modelling shows the second moon around Earth would have been about 1200 km wide. Later, the smaller moon fell back onto the bigger Moon and coated one side with an extra layer of solid crust tens of kilometres thick.

The impact would have been relatively slow, at about 8000 km/h, which is slow enough for rocks not to melt and no impact crater to form. Instead, the rocks and crust from the smaller moon would have spread over and around the bigger moon.

The model may also explain variations in the Moon’s crust, which is dominated on the near side by terrain comparatively rich in potassium, rare-earth elements and phosphorus.

Milky Way Is Now Hidden from One-Third of Humanity
The Milky Way is but a faded memory to one-third of humanity, according to a new global atlas of light pollution produced by Italian and American scientists.

Light pollution is one of the most pervasive forms of environmental alteration. In most developed countries, the ubiquitous presence of artificial lights creates a luminous fog that swamps the stars and constellations of the night sky.

“We’ve got whole generations of people in the United States who have never seen the Milky Way,” said Chris Elvidge, a scientist with the National Centre for Environmental Information. “It’s a big part of our connection to the cosmos, and it’s been lost.”

Elvidge is part of a team that developed a global atlas of light pollution published in Science Advances. Using high resolution satellite data and precision sky brightness measurements, their study produced the most accurate assessment yet of the global impact of light pollution.

“I hope that this atlas will finally open the eyes of people to light pollution,” said lead author Fabio Falchi from the Light Pollution Science and Technology Institute in Italy. The atlas takes advantage of low light imaging calibrated by thousands of ground observations.

Light pollution is most extensive in countries like Singapore, Italy and South Korea, while Canada and Australia retain the most dark sky. In Western Europe only small areas of night sky remain relatively undiminished, mainly in Scotland, Sweden and Norway. Despite their vast open spaces, almost half of the US experiences light-polluted nights.

“Indeed, in the US some of our national parks are just about the last refuge of darkness – places like Yellowstone and the desert south-west,” said co author Dan Duriscoe of the National Park Service. “We’re lucky to have a lot of public land that provides a buffer from large cities.”

Light pollution does more than rob humans of the opportunity to ponder the night sky as the unnatural light can confuse or expose wildlife like insects, birds and sea turtles, often with fatal consequences.

Fortunately, light pollution can be controlled by shielding lights to limit their shine to the immediate area, reducing lighting to the minimum amount needed, or by simply turning them off.

Now isn’t that a bright idea?

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at