StarDate

A total solar eclipse is coming up on Wednesday. The Moon will cover the Sun, turning day to night across a narrow slice of the globe. Parts of Alaska and the northeastern United States will see a partial eclipse.

The timing of the eclipse is known down to the second, and has been for decades; astronomers can predict eclipses far into the future. But making such predictions isn’t easy. It requires a detailed knowledge of the Moon’s orbit around Earth, Earth’s orbit around the Sun, and even the shapes of Earth and the Moon.

Just when the first prediction was made is unclear.

There’s no doubt that people have been predicting lunar eclipses for thousands of years. But they’re easier to forecast. Earth’s shadow is roughly 35 times wider than the Moon’s, so you don’t need to be quite as precise to get it right.

There are stories that the Chinese were predicting solar eclipses more than 4,000 years ago, but no confirmation. An eclipse in 585 BC that stopped a war supposedly was predicted by a Greek scientist. But many present-day scientists doubt that.

The first confirmed prediction was made by Edmond Halley, using the laws of gravity recently devised by Isaac Newton. Halley forecast that an eclipse would cross England on May 3rd, 1715. And he was right. So the event is known as Halley’s Eclipse – honoring the prediction of an astronomical spectacle.

Script by Damond Benningfield

The Moon forms a nice triangle with the twins of Gemini at dawn tomorrow – the stars Castor and Pollux. Castor is to the left of the Moon, with brighter Pollux about the same distance to the lower left.

Castor consists of six known stars. We haven’t found planets orbiting any of them. Pollux is a single star. And it has one likely planet – a giant that’s much heavier than any of the planets in our own solar system.

The planet was discovered 20 years ago, by astronomers at McDonald Observatory. They’d seen evidence of the star earlier, but they couldn’t confirm it.

The planet is known as Pollux b. It also has a formal name – Thestias – a variation of the name of Pollux’s mother in Greek mythology. The planet is almost three times the mass of Jupiter, the Sun’s largest planet. It’s farther from Pollux than Earth is from the Sun. But that’s much too close for the planet to be habitable.

That’s because Pollux has passed beyond the prime phase of life. It’s puffed up to about nine times the Sun’s diameter, so it’s much brighter than the Sun. That extra energy makes Thestias much too hot to have liquid water at its surface – a key ingredient for life.

The temperature could have been more comfortable in the past. The planet itself probably is a big ball of gas – not a good place to live. But if it has any moons, it’s possible that they could have offered better environments for life.

Script by Damond Benningfield

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