Thursday, January 10, 2008

Morning Star

Venus imaged in violet light (NASA)

This one’s for all the morning people. I’m talking about you early birds who are up before the Sun. You need to take a moment this week, step outside with your bracing cup of joe, and greet Venus, the Morning Star.

The convention of referring to planet Venus as both the “Morning Star” and the “Evening Star” began long ago. The ancients, confused by what they observed, thought Venus was two separate bodies: two “wandering stars,” as planets were originally known. Whether we see Venus in the morning before sunrise or in the evening after sunset depends on where it is in its orbit around the Sun-- to the ‘right’ (west) or the ‘left’ (east) of the Sun from our earthly perspective.

Venus is the second planet from the Sun, orbiting between Mercury (closest to the Sun) and Earth (third rock). Venus is only slightly smaller than Earth. Like Earth, Venus is a terrestrial planet, that is, it is composed primarily of rock and metal. And that’s pretty much where the similarity ends.

Lava flows on Venus (NASA)

Thick clouds of sulfuric acid blanket Venus, and its atmosphere is predominantly carbon dioxide. Its barren landscape is arid and covered with old lava flows. Air pressure at the surface is extremely high, comparable to the pressure we would experience ocean diving at 3,000 feet. This dense atmosphere coupled with the cloud blanket raise the planet’s surface temperature high enough to melt lead. An unsuspecting interplanetary tourist would be asphyxiated, crushed, and cooked in one fell swoop.

Venusian cloud cover imaged in ultraviolet (NASA)

But from a safe distance, Venus is an ornament for our sky, beautiful and stunningly bright. I’ve seen punch-drunk amateur astronomers, after a long night of observing, nearly drop equipment in shock when Venus rises, wondering aloud what on Earth that blinding object is! I’ve also heard many an amateur grumble that Venus rising ruins their night vision--in the same way they’d grumble about moonrise destroying their dark adaptation. In fact, after the Sun and the Moon, Venus is the brightest object in our sky.

Why is Venus so darn bright? For one thing, it is the closest planet to Earth, so its apparent size (the amount of sky it covers from our perspective on Earth) is significant. For another thing, its thick cloud cover is extremely reflective, so most of the sunlight it receives bounces back.

Got your steaming cup of joe? Let’s go!

1) Venus will rise in the southeast. If you know about where the Sun rises, watch that spot and you will see Venus rise. If you’re completely flummoxed by cardinal directions, don’t worry. When Venus rises, it will be the brightest object in the sky by far. You can’t mistake it for anything else. It will shine with a blue-white color, unless you have atmospheric murk near the horizon, in which case it may scintillate in a number of colors, like a twinkling star or a UFO.

2) In the continental United States, Venus will rise between 4:30 and 6:00 a.m. on the morning of Friday, January 11, depending on your location. The farther north and west you are in the U.S., the later the rise time. The curve of the Earth hides a slice of southern sky from you folks in the north, and hides a slice of eastern sky from you folks in the west. Therefore, Venus needs to be a little bit higher in the morning sky before she peeks over your southeastern horizon.

3) Each night after the 11th, Venus will rise a few minutes later than the night before. Wait a minute, that’s not how the stars behave! Let’s take a closer look at this Venusian phenomenon.

If you were to track the time that a particular star rises over the eastern horizon--over a series of nights--you would notice that each night it rises a few minutes earlier than the previous night. This makes sense, if you think about how the Earth travels around the Sun in its year-long voyage. Each night, the Earth is a little farther along in its 365-day orbit around the Sun. As a result, our view of the night sky is ever so slightly different--shifted toward our east, the direction in which we’re traveling. Therefore, a star that breached the eastern horizon at a particular time the night before would, at the same time the following night, already have risen. The Earth’s position is constantly changing relative to the panorama of space.

But why, then, is Venus currently rising a few minutes later each night? Aha! Because Venus herself is orbiting the Sun, moving in the same direction as the Earth travels. Venus is outpacing the Earth, moving about 15% faster in a smaller orbit. The result of all this fast living is that Venus is now starting the quarter-leg of her journey when she swings back behind the Sun and, from our vantage point, appears to be getting closer to the Sun. So after we Earthlings rotate for 24 hours and find ourselves again at the same minute we saw Venus rise the night before, she can’t be seen yet. She has moved a little ‘closer’ to the edge of the Sun, is rising over our horizon a little nearer to the time of sunrise, and will eventually disappear behind the Sun for a while.

The mapped surface of Venus (NASA)

4) After you greet the bright goddess in the southeastern sky, turn around and bid farewell to red-faced Mars, hanging low above the northwestern horizon. Now, if talk of the ecliptic still makes you apoplectic, this is a golden opportunity to get some clarity (and serenity) on the concept. With two bright planets in the sky at opposite horizons, you can begin to visualize how the band of zodiacal constellations stretches across the sky. Imagine a curved line connecting the two planets, with the line curving slightly down toward the southwestern horizon. This line that you are imagining approximates the ecliptic, the path that the Sun appears to take against the backdrop of the stars, from our point of view as we circle around the Sun. And since we know from last week’s post that the zodiac is a daisy chain of constellations that, along with the Moon and the planets, appears to hug the ecliptic, voila! You can see that having a couple planets in the night sky can really help with sky orientation.

5) You know, you’ve been such a great audience, I’m going to throw in a bonus planet. With your left shoulder pointed toward brilliant Venus and your right shoulder pointed toward ruddy Mars, you’ll be facing southwest. About halfway between the zenith (the point directly above your head) and the southwestern horizon, you will see a bright, golden-colored “star.” This is the ringed planet Saturn, currently spending a little quality time in the zodiacal constellation of Leo the Lion and audaciously close to the ecliptic.