StarDate

Mars and the Moon hang out in tomorrow’s early-morning sky. Mars looks like a bright star to the right of the Moon at first light.

The Moon is a thin crescent, and it’s getting thinner by the day. That means the Moon is about to cross between Earth and the Sun, so the Sun is lighting up less of the lunar hemisphere that faces our way. But you can make out the nighttime part of the disk because it’s lit up by earthshine – sunlight reflected from Earth.

If anyone were standing on that part of the Moon, the earthshine would be bright enough to read by. That’s because Earth is in its gibbous phase – it’s almost fully in daylight. Earth is much bigger than the Moon, and its surface is much more reflective. So a gibbous Earth is dozens of times brighter than a gibbous Moon.

From Mars, both Earth and the Moon appear gibbous. That’s because Earth and Mars line up more or less on opposite sides of the Sun. But over time, Earth and the Moon would appear to go through a full cycle of phases, just as the Moon does as seen from Earth. And their phase would always be the same.

As the Moon orbits Earth, the angle between them would vary. So sometimes, Earth and the Moon would appear so close together that they’d form a single bright point. But at their widest separation, it would be easy to see them as individual points, with Earth especially bright, and the Moon a good bit fainter.

Script by Damond Benningfield

The terms that describe the phases of the Moon can be a little confusing. Take tomorrow morning, for example. From most of the United States, the Sun will light up one-quarter of the lunar hemisphere that faces Earth. You might think that would make it a “quarter” Moon – but no.

A “quarter Moon” doesn’t describe the Moon’s appearance – it describes the position in its month-long cycle of phases.

The cycle begins when the Moon is new. The Moon crosses the line between Earth and the Sun then, so it’s hidden in the Sun’s glare. A bit more than a week after that, the Moon is a quarter of the way through the cycle – it’s at first quarter. It lines up at a right angle to the Earth-Sun line, so the Sun lights up half of the visible disk.

A week later comes full Moon. And a week after that comes last quarter – three-quarters of the way through the cycle. Once again, half of the lunar disk is illuminated – but it’s the opposite half from the first-quarter Moon. Then the Moon rolls back around to new – starting a new cycle.

The next cycle starts on Wednesday. And this time it brings a rare treat – a total solar eclipse.

In the meantime, look for the Moon passing through Taurus tomorrow. Aldebaran, the bull’s bright eye, is to the right of the Moon. Elnath, the tip of one of its horns, is closer to the lower left of the Moon. As a bonus, Mars is below them, and the Moon will pass close to it on Sunday.

Script by Damond Benningfield

The Dog Star is just beginning to emerge from the Sun’s glare in the dawn sky. By tradition, that means we’re just getting into the Dog Days of summer.

Sirius is the brightest star in the night sky. It’s climbing into view in the morning glow of the brightest star in all the sky – the Sun. Sirius is quite low in the east-southeast shortly before sunrise. It’s best seen from the southern tier of states, although it’ll pull into better view from the northern states over the coming days and weeks.

Sirius is also the leading light of the constellation Canis Major, the big dog, so it’s known as the Dog Star. And that’s where we get the name for this part of summer – the Dog Days.

Sirius makes its closest approach to the Sun during summer, disappearing for a few weeks in the Sun’s glare. In ancient times, it returned to view in the morning by early July – just as the weather was reaching its hottest. At the time, it seemed logical to associate the bright star with the sweltering weather. So the people of ancient Greece and Rome named that stifling time of year in the star’s honor.

Thanks to an effect known as precession, the stars shift position with respect to the Sun over the centuries. So Sirius now returns to view a few weeks later than it did thousands of years ago. So the heat is already in full force at northern latitudes – the Dog Days of summer.

Script by Damond Benningfield

NGC 6397 seems like a pretty ordinary globular star cluster. It’s not especially big or bright, and nothing about its stars is going to strike anyone as odd. Yet it’s been an important astronomical laboratory. Among other things, it’s helped astronomers determine the age of the galaxy, and the minimum “weight” of a star.

NGC 6397 is in the constellation Ara, the altar. It’s below the tail of Scorpius, the scorpion. That puts it so far south that it’s visible only from the southernmost United States.

Hubble Space Telescope has monitored the cluster for decades. It contains several hundred thousand stars, packed into a ball only a few dozen light-years across. The Milky Way contains about 150 of these clusters, which are the galaxy’s oldest inhabitants.

Over the decades, Hubble has monitored many of the cluster’s stars. The observations helped astronomers lock down the minimum mass for an object to shine as a true star – about eight percent the mass of the Sun.

Another set of observations helped measure the cluster’s distance – just 7800 light-years – one of the closest of all globulars. That number is important for determining the cluster’s age – 13.4 billion years. That means that NGC 6397 – and its home galaxy, the Milky Way – took shape when the universe was just 400 million years old.

Script by Damond Benningfield

The gods of ancient Greece weren’t great parents. One of the ancient gods known as the Titans, for example, swallowed his newborn children. Eventually, though, the kiddos escaped and overthrew him. Their victory is commemorated in the stars, as the altar where they swore allegiance to each other. It’s the constellation Ara, and it’s below the tail of the scorpion.

In the story, Cronus was the king of the Titans. He’d come to power by defeating his own father. A prophecy said that one of his children would defeat him. To prevent that, he swallowed each of his first five children.

But his wife, Rhea, got tired of that plan. When the last child was born, she hustled him away to an island and gave Cronus a rock wrapped in a blanket.

That child was Zeus. When he grew up, he returned home and forced his dad to cough up his brothers and sisters. They pledged to defeat the Titans, and won a 10-year war. Zeus became the king of the new gods and ruler of the sky. To celebrate the victory, he placed the altar among the stars.

The altar is so far south that most of it’s visible only from far-southern parts of the United States – places like Florida, South Texas, and especially Hawaii.

Ara is one of the smaller constellations, with only a couple of moderately bright stars. But it does have some good deep-sky objects, and we’ll talk about one of them tomorrow.

Script by Damond Benningfield