The Sun’s closest planet is making a pretty good appearance in the early evening. As seen from Earth, it’s just about as far from the Sun as it ever gets. It looks like a bright star low in the west-northwest beginning shortly after sunset.

Mercury is getting ready to cross between Earth and the Sun – a point known as inferior conjunction. In the meantime, it’ll drop closer and closer to the Sun as viewed from Earth. It’ll vanish in the twilight in a couple of weeks.

Mercury’s orbit is carrying the planet closer to Earth. Tonight, it’s about 78 million miles away. At conjunction, it’ll close to just 53 million miles.

On average, Mercury returns to conjunction every 116 days. But the gap varies by more than 10 days in either direction. That’s because Mercury’s orbit is more lopsided than that of any other planet. When Mercury is closer to the Sun it moves faster; when it’s farther it moves more slowly. So the gap between conjunctions varies depending on where Mercury is in its orbit when it passes Earth.

For the next few nights, look for Mercury as evening twilight fades. Although the planet is bright, it’s so low that you need a clear horizon to spot it. It lines up to the lower right of the much brighter planets Venus and Jupiter. And the crescent Moon will join the lineup on Tuesday, helping point the way to the little planet.

Script by Damond Benningfield

A relic from the early universe is racing through Libra. It’s moving across the constellation at 800,000 miles per hour. That’s far faster than most of the stars around us. So it’ll move out of Libra in the blink of a cosmic eye.

The star is HD 140283. But it also has a nickname – the Methuselah Star. That’s because it’s probably about 13 billion to 14 billion years old. Since the universe itself is only 13.8 billion years old, that makes the star one of the oldest anywhere.

Astronomers have pieced together several bits of evidence to arrive at that age. The key bit is the star’s composition. It has very low levels of elements that are heavier than hydrogen and helium, the simplest elements. Heavier elements were created in the hearts of stars, then spewed into space when the stars died. Some of these elements then were incorporated into later generations of stars.

HD 140283 has less than half a percent as much iron as the Sun, with slightly higher levels of oxygen and a few other elements. Those abundances tell astronomers the star must have been born when the universe was brand new.

As night falls, the star is to the lower left of Zubeneschamali, Libra’s leading light. It’s too faint to see with the eye alone, but it is visible through binoculars.

Script by Damond Benningfield

Every star in the night sky is moving – orbiting the center of the galaxy. Some are moving toward us, while others are moving away. We can’t see that motion because the stars are so remote. But we can measure it with special instruments – one of the most important techniques in astronomy.

The instruments break the light of a star or other object into its individual wavelengths or colors. Each chemical element imprints its own “barcode” in that array of wavelengths. A star’s motion toward or away from us causes the barcodes to shift position.

If it’s moving away from us, the shift is toward longer, redder wavelengths: a redshift. And if it’s moving toward us, the shift is toward shorter, bluer wavelengths: a blueshift. The size of the shift reveals the speed.

The technique also can reveal how fast a star is spinning; the side that’s rotating toward us is blueshifted, while the opposite side is redshifted. And it can reveal orbiting companions; their gravity pushes and pulls the star, slightly changing its motion.

Two bright stars with well-measured shifts are in view as the sky darkens this evening. Regulus is in the west, well to the upper left of the brilliant planets Venus and Jupiter. Its light is redshifted; it’s moving away from us at about 9300 miles per hour. Antares, quite low in the southeast, is sliding toward us at about 8600 miles per hour – giving its light a definite blueshift.

Script by Damond Benningfield