It’s hard to look away from a stunning sunset. If you turn around, though, you might see another colorful show, although one that’s more subdued than the main event.
As the Sun drops below the western horizon, Earth’s shadow climbs into view above the opposite horizon. It forms a dark blue band that climbs higher as the evening progresses. It’s also visible in the dawn sky, shortly before sunrise. The band forms a thin wedge where it intersects the horizon, and grows thicker as it approaches the point directly opposite the Sun.
At the same time, a band of bright pink appears above the shadow. It’s known as the anti-twilight arch. It’s created when the red hues of sunlight strike particles in the atmosphere on the opposite horizon, causing them to shine reddish pink. It’s also known as the Belt of Venus. And right now, Venus – the “evening star” – is directly opposite that band in early evening.
As the fuzzy line that separates the two bands climbs higher, it gets even fuzzier. Soon, it reaches a height where the atmosphere is thinner, so less red light is scattered there. The pink belt fades to blue, and both the Belt of Venus and Earth’s shadow merge into the darkening night.
Earth’s shadow extends far into space. When the geometry is just right, the Moon passes through the shadow, creating another beautiful show: a lunar eclipse. And an eclipse is coming up tomorrow night. We’ll talk about that on our next episode.
Script by Damond Benningfield
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2:20
Moon and Regulus
Hundreds of valleys meander across the Moon. Many of them look like river valleys on Earth. And some of them might have been formed in the same way – by flowing liquid. There’s no water on the Moon, though, so the channels were carved by lava.
The channels are known as rilles – from a German word that means “grooves.” They come in three basic forms. One form probably took shape as pieces of crust pulled apart, leaving a wide, straight gap between them.
The second category follows a gently curving path. These rilles probably formed when a river of lava cooled and condensed, sinking into the ground.
The final group looks most like Earthly riverbeds. “Sinuous” rilles twist and turn across the surface – sometimes dramatically. They probably formed from flowing lava. The lava either carved a channel on the surface or tunneled below the surface, and the empty tube later collapsed. Sinuous rilles often begin at a crater, which probably is the “vent” where lava poured onto the surface.
Apollo 15 landed near one of these rilles. Known as Hadley Rille, it’s about 75 miles long, a mile wide, and a thousand feet deep. Astronauts looked into its depths and gathered samples from its rim – the edge of a “river valley” on the Moon.
The almost-full Moon is in the east at nightfall. Regulus, the brightest star of Leo, is close below it. They move closer during the night, and are almost touching as they set, before dawn.
Script by Damond Benningfield
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2:20
Venus and Mercury
The Sun’s closest planets are appearing close to each other in the early evening sky. Venus is the brilliant “evening star.” Mercury is close to its lower left this evening, but will slide above Venus over the next few nights.
Mercury is the Sun’s closest planet. It’s also the smallest planet – a dense ball of rock and metal about half-again the diameter of the Moon.
At its equator, noontime temperatures climb to about 800 degrees Fahrenheit. But some deep craters near the poles never see daylight. So temperatures there hit a couple of hundred below zero.
That’s not the case on Venus – not even close. Even though Venus is tens of millions of miles farther from the Sun, it’s much hotter. That’s because Venus is more massive than Mercury, so its surface gravity is stronger. That’s allowed Venus to hold onto its atmosphere.
Heat from the Sun has baked gases out of its rocks, making the air especially thick – the surface pressure is about 90 times the pressure on Earth. And the atmosphere is made mainly of carbon dioxide, which traps heat, preventing the heat from escaping into space. So the average temperature across the planet is about 865 degrees. It’s blazing hot even at the poles. The only places that aren’t that hot are mountaintops – the cool spots on the solar system’s hottest planet.
Venus and Mercury are quite low in the west during evening twilight. They’ll stand side by side in a couple of days.
Script by Damond Benningfield
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2:14
Pole Stars
The North Star is one of the most important beacons in the sky. It serves as a compass, pointing the way due north. And it also serves as the hub of the sky – all the other stars appear to circle around it as Earth turns on its axis.
More than 4,000 years ago, the architects of Egypt used the North Star to align the pyramids of Giza. And more than four decades ago, Apollo astronauts used it to help guide them to the Moon. But the two groups were aided by different north stars. The Egyptians used a star called Thuban, in Draco, the dragon. The astronauts used Polaris, at the end of the handle of the Little Dipper.
Thuban didn’t explode or fade away – it’s still in plain sight. Instead, Earth’s axis turned away from the star – an effect called precession.
It’s caused by the gravitational tug of the Sun and Moon, which cause our planet to wobble like a spinning top. As it wobbles, the axis points toward different stars. Four thousand years ago, it aimed at Thuban. Today, it aims at Polaris.
Over the next century, the pole will take slightly better aim at Polaris. Right now, the star is about two-thirds of a degree from where the axis is pointing. Around the year 2100, it’ll be less than half a degree from that spot.
After that, though, the pole will move away from Polaris. By around the year 4100, it’ll take aim at a star that’s one constellation over, in Cepheus – a North Star to guide future generations.
Script by Damond Benningfield
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2:14
Moon and Mars
Follow the water is NASA’s mantra for Mars exploration. Water is a key ingredient for life. So missions to the Red Planet have sought out regions that show evidence of water, either past or present. Such regions should be the most likely to hold evidence of life.
But some scientists think the search ought to be expanded. Don’t just follow the water – follow the salt.
The Viking landers of the 1970s carried several experiments to look for microscopic life. They fed a bit of Martian dirt into small chemical laboratories. The experiments added water, nutrients, or other ingredients. They then looked for gases or radioactive elements that might be produced by such organisms.
A couple of the experiments found just what they were looking for. But most scientists later decided that the evidence was produced by chemical reactions. Others maintained that they really were produced by life.
A recent paper argued that the composition of the soil at the landing sites was very salty. On Earth, some organisms live in such conditions. They draw their water from the salts. But if you add water, they die. So the Viking landers might have found life – then killed it off. So NASA might need to expand the search – by following the salt.
Mars is especially easy to see tonight. It’s right next to the Moon at nightfall, and looks like a bright orange star. They’ll be a little farther apart as they set, in the wee hours of the morning.
Script by Damond Benningfield
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