Stargazers: Mercury and Venus visible just after sunset

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If you have a low horizon to your west, you may have already spotted a couple of bright "stars" appearing in that direction just after sunset. Well, those aren't stars, but planets.

After many months of seeing no planets in our evening sky, 2018 will give us quite a planetary show. Later this summer, Jupiter, Saturn and Mars will all make an appearance, but it all begins this month with Venus and Mercury at sunset.

Venus, of course, is a good place to start. Not long after the sun disappears, you'll see it — the brightest planet of all.

Venus appears so dazzling because it is a world roughly the size of our own Earth and is shrouded by thick white clouds that reflect back into space two-thirds of all sunlight that falls on them.

If you aim a small telescope in the direction of this planet, you'll now see only a tiny white disk. That's because Venus is on the other side of the sun — about as distant as it can ever become and directly lit by sunlight. Give it a few more months, however, and this planet will swing around to our side of the sun, appearing brighter and higher in our western sky at dusk, and taking on a quarter phase by summer and a crescent shape by autumn.

For the next week or so, the planet Mercury will appear as a much fainter "star" just to the upper right of Venus. Don't be surprised if it appears to twinkle; the thick column of air through which its light must pass will do that even to a planet, despite what we might have learned in grade school.

If you've never seen Mercury, you're certainly not alone. Many stargazers, including the great 16th-century Polish astronomer Nicolaus Copernicus, have never spotted it. But since this week offers a great opportunity, now's the time to check out this elusive planet.

I call Mercury "elusive" because it whips around the sun in only 88 days and, as a result, never remains in our sky for long or strays far from the sun's glare. Even under the most ideal conditions, we can never see Mercury more than 28 degrees from the sun, which means it can never appear in a completely dark sky.

If your western sky is clear, try aiming a small telescope toward Mercury, but don't expect to see much. Mercury is relatively small, some two and a half times smaller than the Earth, and its light will be significantly distorted by air turbulence near the horizon. But if you're fortunate enough to see a steady image, Mercury will now appear in a first-quarter phase. Keep watch over the next week or so and you'll see it gradually evolve into a crescent before vanishing in the sun's glare near the end of the month.

If you have trouble spotting these planets, on Sunday, March 18, the (very) thin crescent moon will create a beautiful sight just to the left of this pair after sunset. The following night, the thicker crescent moon will appear much easier to spot above and to their left.

     Visit Dennis Mammana at www.dennismammana.com.


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