Planet Watch

This is the best time to be a planet watcher. Within two weeks, you’ll be able to see all the “naked eye” planets (Mercury, Venus, Mars, Jupiter and Saturn) in the evening sky. Presently, you can see four of them. (Mercury is the only absent planet.)

Mercury is not visible right now. This small planet will be moving into superior conjunction (on the other side of the Sun with respect to Earth) early this month before it emerges into the western evening sky. April’s apparition will be Mercury’s best for 2002.

Venus is low in the southwestern evening sky. Venus sets more than an hour and a half after the Sun sets.

Mars sets about four hours after the Sun. You will find the red planet in the southwestern sky. Mars will be about 130 times dimmer than its western evening sky companion, Venus.

Jupiter is high in the western sky by the end of evening twilight. The giant planet will be the “first star” to appear after sunset. Jupiter resides at the “foot of Gemini,” at the western edge of the constellation Gemini, the twins. Gemini is represented by two starlines to the northeast of Orion. Jupiter marks the point at the foot of the northern twin, Castor.

Saturn is north of Aldebaran, the brightest star in Taurus the Bull. Saturn is not as bright as Jupiter, but it will be easy to spot within the v-shaped constellation Taurus. Both the bull and the planet Saturn will be well into the western sky by the onset of darkness.

What gravity isn’t

Gravity is the one fundamental force with which we are the most familiar. It is the force which enables us to remain on this planet without being propelled into outer space.

Yet, as you stand or sit there, you might wonder, “What exactly is the nature of gravitation?” Or, what is it about gravity that enables it to keep me right here on Earth? As simple as that question sounds, it has no easy answer.

Even though humans have been aware of gravity since time immemorial (at least since the first person tried to fly and failed), the idea of gravity was misunderstood until the time of Galileo and Newton.

Ancient peoples believed that the physical world was governed by two main forces: levity – the tendency of light objects to rise, and gravity – the tendency of heavy objects to fall. This perception is based upon the observation that light objects like feathers appeared to swoop and dodge under the influence of even the lightest breezes, while rocks and bricks fell to the ground without impediment.

Galileo was the first to demonstrate that the rate at which objects descend to Earth is independent of the object’s masses. If they fell in a vacuum, a feather and a rock would accelerate toward Earth at the same rate. The swooping and dodging of a falling feather is a consequence of aerodynamics. Remove the air and gravity becomes the sole force acting upon that stone.

Isaac Newton based his law of gravity partially on Galileo’s work. He modeled gravity as an attractive force between all massive objects. The magnitude of the force between the objects is proportional to the masses of the objects and inversely proportional to the square of the separation distance between the objects. (For instance, if you double the distance between two asteroids, the force between them will be reduced to a quarter of its original value. Triple the distance, the force is reduced to one-ninth its original value.)

Newton’s highly mathematical description of gravity proved to be a powerful tool for understanding how objects behaved under its influence – from stones on cliffs to the orbit of the Moon.

Yet, despite the brilliance of his work, Newton conceded that he did not understand the concept of “action at a distance.” He even said once that in regard to this phenomena he couldn’t see how any man of “rational philosophy” could fall into it (no pun intended).

In other words, Newton didn’t know what made massive objects attract each other. He just knew that they did and he was able to formulate a mathematical model based on this attraction.

Albert Einstein offered a partial answer to the nature of gravity with his general theory of relativity (1915). Einstein described gravity as the distortion of space-time caused by massive objects.

For instance, the Sun distorts its local geometry of space-time, creating a gravity well. The planets are caught within that well, preventing them from escaping. Our planetary orbits are created by the Sun’s indentation of Space. Similarly, the Moon is trapped within Earth’s gravity well.

The best way to imagine the distortion of space-time is to think of a taut rubber sheet. If you put a bowling ball on that sheet, the ball will distort the area around it, creating a dent. If you roll a lot of marbles across the sheet, a few of those closest to the dent will be drawn into it. In this manner, massive objects will be captured by larger bodies if they venture too close to them.

So, in effect, Einstein explained that the force of gravity isn’t a force at all: it is a distortion in space caused by a massive particle, although he didn’t address the actual issue of space, itself (What is it that is distorted? Why is it distorted? What is it about massive objects that causes the distortion?)

So, as much progress as Einstein made, he still left us with unanswered questions about the nature of gravity. Of course, with his description of gravity, he showed us what gravity isn’t. Quite honestly, that is some progress, indeed.


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