Moon, Mars, and stars tell tale of increasing precision

The moon this evening shines with a companion. Look southwest at dusk (assuming the sky has cleared), and there the half-lit moon will be hanging with a bright, orange-yellow fire-spark just above it. That’s the planet Mars. The moon and Mars will appear striking enough to attract attention from people who just glance up. You can tell them what they’re seeing.

Other facts put this scene in 3-D perspective. Mars is actually twice as large as the moon. The reason it looks so small in comparison is it’s 400 times farther away.

A little to the left of this pretty pair shines a star nearly as bright as Mars. It’s Spica in Virgo: a blue-white-hot nuclear reactor 1,500 times larger than Mars and 16 million times farther away. Space is big.


Our neighbor moon, so nearby and orbiting Earth, is always on the move against the celestial background. At dusk we’ll see the moon at its closest to Mars, but within an hour or two you’ll see it drawing away to the fire-spark’s left. Its configuration with Mars and Spica continues changing until the whole group sets after midnight.

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The real show, however, happens to our south. Skywatchers in Panama and much of South America will see the moon go right across Mars. The moon’s dark edge will creep up to the little orange dot, then fade it from view over the course of about half a minute as the moon covers it up. About an hour later Mars will reappear from behind the moon’s other side. This remarkable celestial event is called an occultation, from the Latin for “hide.”

Seven hours later, the moon will skim below Spica, but there won’t be an occultation visible from anywhere on Earth.

Often the moon does occult stars, though, and these events are dramatically different from what will happen to Mars. A star doesn’t fade slowly behind the moon’s edge but snaps out instantly. This is because stars are so far away that they appear as fantastically tiny pinpoints. The actual, sun-like balls typically appear tens of thousands of times too tiny for even the sharpest-eyed person to resolve their round shapes. They’re too small even for the largest telescopes to resolve without fancy tricks.

This, and their great distance, make stars extremely accurate reference points for measuring positions in the sky. And that useful resource is something people have exploited for all sorts of purposes since ancient times.

A star to steer by


Let’s start with timekeeping. The sun was handy to use for this but its apparent motions, caused by our own moving viewpoint on Earth, are complicated. Star sightings worked better than sundials. As accurate timekeeping really began to matter, nations set up observatories with telescopes designed for timing when a star crossed the local north-south line to split-second accuracy – as fast as the reflexes of the astronomer watching through the telescope’s eyepiece.

If you wanted to rule the world, you needed ships that knew their position. Latitude was easy to find at sea, but longitude required an excellent clock on board set to the standard time of a known place. So in 1675, England established the Royal Greenwich Observatory to time stars. Eventually, its staff rigged a large ball on a tower on the building’s roof and dropped it at exactly 1 p.m. every day, so that navigators watching from the harbor could flick the start lever on their chronometers at the correct instant.

That’s why the world’s standard time is still called Greenwich Mean Time. It’s why zero degrees longitude was defined by the location of the Greenwich star-timing telescope, and why we still watch a ball drop to mark midnight on New Year’s Eve even if few remember why.

Nowadays, atomic clocks keep time much better than the turning Earth. So when you set your watch, you’re not doing it according to some astronomer’s reflexes at the eyepiece, but by the vibrations of cesium, hydrogen, and rubidium atoms in a bank of devices at the US Naval Observatory in Washington, D.C.

Still, stars have their place. Spacecraft use them. As late as the 1980s, the Naval Observatory’s ever-improving star positions were being used to help give America’s nuclear missiles the ability to launch a surprise first strike and, in theory, win a nuclear war. This required pinpoint targeting to destroy every enemy weapon before someone on the other side could push the button.


That use of star positions is presumably obsolete now. Even your car’s GPS provides first-strike precision.

But as ever-refined star positions lose their importance on Earth, they are increasingly valuable to the higher pursuits of pure astronomy. Last year, the European Space Agency launched a craft named Gaia that is now mapping a billion stars far more accurately than ever. Its overwhelming data should set off revolutions across many areas of astronomy. Its positions will provide, indirectly, the first really accurate distances to most stars. Without knowing a sun’s distance you don’t really know its true brightness, size, energy production, or much else about it. One of the oldest types of astronomy will take on new and probably unexpected roles.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge ( His Star Watch column appears the first Saturday of every month.