If there ever was a planet that has gotten a bad rap for its inability to be readily observed it would have to be Mercury, known in some circles as the "elusive planet."
In his book "The Solar System and Back" (Doubleday, 1970), famed science writer Isaac Asimov (1920-1992) argued that the planet Mercury:
"… is hardly ever visible when it is truly dark. Mercury … will be seen only near the horizon in dawn or twilight, amid haze and sun glare. I suspect, in fact, that many people today (when the horizon is dirtier and the sky much hazier with the glare of artificial light than it was in centuries past) have never seen Mercury."
Nonetheless, during this week we will be presented with an excellent opportunity to view Mercury in the early morning dawn sky. Mercury is called an "inferior planet" because its orbit is nearer to the sun than the Earth's. Therefore, it always appears from our vantage point (as Asimov indicated) to be in the same general direction as the sun.
Mercury's major extremes
The planet is well named for it is the closest planet to the sun and the swiftest of the sun's family, averaging about 30 miles per second (48 kilometers per second); making its yearly journey in only 88 Earth days. Interestingly, the time it takes Mercury to rotate once on its axis is 59 days, so that all parts of its surface experiences periods of intense heat and extreme cold. [Latest Photos of Mercury From NASA]
While Mercury's average distance from the sun is only 36 million miles (58 million km), Mercury experiences by far the greatest range of temperatures: nearly 900 degrees Fahrenheit (482 Celsius) on its day side, minus 300 F (minus 184 C) on its night side.
Mercury is currently rising before the sun this week and is surprisingly easy to see from now through Sept. 15. All you have to do is just look low above the eastern horizon during morning twilight, from about 30 to 45 minutes before sunrise for a bright yellowish-orange "star."
On the date of its greatest elongation, Mercury appeared roughly half-illuminated and the amount of its surface illuminated by the sun has continued to increase with each passing day. So although it has now been progressing closer to the sun's vicinity, it will continue to brighten steadily, which should help keep it in relatively easy view over the upcoming week.
Mercury and Regulus will be separated by three-quarters of a degree. For comparison, the apparent width of the moon is 1/2 of a degree, while your fist held at arm's length covers a 10-degree patch of the night sky.
By Sept. 15 it will have brightened substantially, surpassing the brilliance of every star in the sky with the exception of Sirius (the brightest of all stars). But by then it will be rising barely an hour before the sun and despite its great brilliance, will likely be swallowed up by the dawn twilight glow in the mornings that follow.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.
