|
Facts of the Matter Richard Brill |
Catch Mars’
brilliance now or
wait 60,000 years
"Once in about every fifteen years a startling visitant makes his appearance upon our midnight skies -- a great red star that rises at sunset through the haze about the eastern horizon, and then, mounting higher with the deepening night, blazes forth against the dark background of space with a splendor that outshines Sirius and rivals the giant Jupiter himself."Between now and Aug. 27 we will witness the rarest and one of the most brilliant recurrences of the cosmic spectacle that inspired that comment, written by astronomer Percival Lowell in 1895.
Lowell, who devoted his life to its observation and study, was referring to the planet Mars and the event that astronomers refer to as "perihelic opposition." It occurs when Earth and another planet when are lined up on the same side of the sun. It is called opposition because the other planet and the sun will be on opposite sides of the earth, and in opposite directions in the sky as seen from Earth. It is perihelic because it occurs when the planet is at its closest to the sun and also closest to Earth.
Earth and eight other planets orbit the sun. The further from the sun a planet is, the longer it takes a to complete one revolution. So, the inner planets continually "lap" the outer planets, with opposition occurring whenever one catches up with a more distant neighbor. Earth is closer to the sun than Mars so it moves faster, overtaking and passing Mars every 780 days.
Around opposition, two noteworthy things happen: Mars goes retrograde and it appears much larger and much brighter.
Normally Mars moves against the star background from east to west, rising and setting about two minutes earlier every day. But a month or so before opposition it slows and eventually reverses its daily motion for a couple of months before reversing once again and continuing in its normal direction.
The second, and less subtle, occurrence -- and the one alluded to by Lowell -- is that Mars' apparent size increases and it becomes dramatically brighter as Earth catches up to it.
At every opposition Earth and Mars are close, but not all oppositions are the same. A typical opposition may well go unnoticed, but when it occurs at Mars' closest approach to the sun, it becomes a spectacular show. It is also the best time to study the planet, and perihelic oppositions have historically proved to be most fruitful for gaining information about it. This year is no exception, with three spacecraft already heading toward Mars for rendezvous later this year.
The striking feature of the perihelic opposition of Aug. 27 is that it brings the two planets about as near to one another as they can get. There have been six very close perihelics in the past 350 years but this will be the closest the two planets have been for 60,000 years. The last time they were anywhere near this close was in 1924.
The reason for the variation in the approach distance is in the orbital dynamics of the two planets. The orbits of all planets are not circular but elliptical. An ellipse is a geometrically flattened circle; the degree of flattening is measured by the eccentricity of the orbit, which translates to a "percent of flattening." The sun is not at the center of the ellipse, but rather is slightly off center by an amount that is proportional to the eccentricity, with the result that the planet's distance from the sun changes continuously throughout the orbit.
Most of the planets are in orbits with very small eccentricity. The eccentricity of the orbit of Venus is 1 percent, Earth's is 2 percent, but Mars' is 9 percent. Because of its more eccentric orbit Mars' distance from the sun varies by more than 25 million miles, compared to Earth's barely 6 million mile difference.
Opposition can occur when the two planets are anywhere in their respective orbits, but only one particular configuration brings them the closest together. That occurs when Mars is near perihelion (closest to the sun), hence the adjective, perihelic.
Earth passes the perihelion point of Mars late in August every year, but since the time between oppositions is longer than the Martian year, the point of opposition migrates around the orbit of Mars each year. It takes anywhere from 15 to 17 years for it to come back near the perihelion point, but much longer than that to come back to the same point. To complicate things even more, the orbits of the two planets do not lie in the same plane and the eccentricities of both orbits change at different rates as they are perturbed by small tidal forces exerted by the other planets, most notably Jupiter.
What this all boils down to is that the distance between Earth and Mars at opposition can be anywhere from 34 to 64 million miles, while at its furthest Mars is nearly 250 million miles away. So, its apparent size varies by a factor of seven and at its brightest Mars shines 75 times brighter than at its dimmest.
Its white polar ice cap is visible through a good backyard telescope now; its apparent size will nearly double between now and Aug. 27 and it will become almost four times brighter as we speed toward it at 25,000 mph. So if you notice a fiery, red "star" that looms on the eastern horizon just after sunset and gets bigger and brighter night after night, don't be surprised, be awed; it's our nearest neighbor putting on its best show in 600 centuries!
We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at rickb@hcc.hawaii.edu