Twinkle Twinkle Little Star is probably the first scientific fact babies hear in the English-speaking world. It’s easily verifiable too. You look out of the window on a clear night and you will see stars twinkling. But not everything twinkles in the night sky, namely the planets.
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There are five planets visible to the naked eye in the sky: Mercury, Venus, Mars, Jupiter, and Saturn. Venus and especially Mercury do not stray too far from the apparent position of the Sun. As their orbit is inside Earth’s own, there is no geometrical arrangement for them to get very high from the horizon, so you’ll find them in the east or west depending on where they and the Earth are in their respective orbits.
The other three planets are all further from the Sun than Earth, so they can get to the zenith, the highest point in the sky – but you can always find them around the ecliptic. This is the imaginary plane in which the Earth orbits the Sun, and no planet is too many degrees off that. Finding that line is easy, but even easier is to find the star-like objects that don’t twinkle, as you do not need any knowledge of cardinal directions to do so.
Stars twinkle because of the atmosphere. Even on the calmest of days, with no ground wind, there will be motions and turbulence in the 100 kilometers (62 miles) between the ground and space. Stars are, for all intents and purposes, point sources, so this turbulence shifts the light that we get from the stars ever so slightly, causing them to twinkle.
Planets might appear to our eyes as equally tiny points, but they are close enough that they are actually little disks. Given the extended size of these disks, the turbulence of the atmosphere doesn’t affect them as much, and so their light appears not to twinkle – making them very distinct in the sky compared to stars.
Turbulence in the atmosphere is actually a major drawback of high-precision ground-based astronomy. Some observatories use lasers to create fake stars that allow them to correct the images. Observatories can have adaptive optics whose mirrors move in response to turbulence. The Extremely Large Telescope will use a mirror that tips and tilts 10 times per second to correct atmospheric blurring.
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If you want a practical exercise to see if you can recognize planets among the stars, you have picked the perfect time. There’s currently a planetary parade – with all the planets but Mercury in a line. And in a few weeks, Mercury too will be visible. So go out after sunset, and try to spot them!
Source Link: Why Do Stars Twinkle But Planets Don’t?
