MOON-WATCH

The Moon was full on Jan. 28. The full moon rises around sunset and will be visible throughout the entire evening. The moon reaches the last quarter point on Feb. 4 and will be new on Feb. 12.

PLANET-WATCH

Mercury vanishes into the solar glare and will not be visible until next month.

Venus is a bright object, but it is not observable this month because it is in superior conjunction with the Sun, which is much brighter.

Venus will emerge from the twilight late this month, but will be difficult to see until mid to late March.

Mars will not be a spectacular object for us in 2002, because the

red planet will not be in opposition again until 2003. Mars will be in the southwestern evening sky and will set by mid to late evening in January.

Jupiter is brilliant. Jupiter will remain in our evening sky until late spring when it will set with the Sun. Jupiter is at its brightest of the year right now.

You can observe Jupiter’s apparent brightness diminish during the next few months as its distance from Earth increases.

Saturn is high in the eastern evening sky and will set in the pre-dawn western sky. Saturn resides in Taurus the Bull.

SPHEROIDS

Time for a confession: When small children ask us about the shape of planet Earth, we tend to lie.

Or, more correctly, we approximate. We tell them that Earth is a sphere, or a ball. Truth is, Earth is not a perfect sphere. It is an oblate spheroid. (Or, for those who demand high precision, it is a geoid which approximates to an oblate spheroid. How do you think a 5-year old would react if they were told that?)

Think of this shape as a sphere with a bit of an extra “bulge” around the center.

The bulging effect is a consequence of Earth’s rotation: the rotational motion tends to push out, and distorts the region around the planet’s equator. The degree to which this bulge distorts a planet’s spherical shape is called the “oblateness factor”.

As you might suspect, this value is determined, at least in part, by a planet’s rotational speed. But other factors, such as the gravitational pull of nearby bodies, planetary densities also must be considered.

Earth’s oblateness is 1/298 (the difference between its polar and equatorial diameter, not a dramatic difference).

If one were to survey the solar system, it would become apparent that the most oblate planets are to be found in the gas giant realm.

The fact that the gas giants (Jupiter, Saturn, Uranus and Neptune) are more oblate than the terrestrials (Mercury, Venus, Earth and Mars) is sensible. Gas giant worlds rotate very quickly. All of them complete a rotation in less than one Earth day.

Also, as their name suggests, they are primarily gaseous spheres. The rigidity of the terrestrials counteracts some of the rotational bulging effects. Whereas, the fluid gas atmospheres in the gas giants are molded more dramatically by their spin.

Think of it this way: when a pizza chef spins dough on his fingertips, the dough changes shape easily. How much luck would the chef have trying to alter the shape of a platinum disc using this method?

Saturn is the most oblate planet (1/10.2). It rotates once every 10 hours, 40 minutes. It is also the least dense planet of them all. Combine these factors, and you can understand why Saturn has such a bulge.

Mercury, Venus and the Moon have zero oblateness. This “lack of bulge” is not surprising once you find out how slowly these worlds rotate. Mercury rotates once every 58.6 days; Venus, once every 243 days; Moon, once every 27.3 days.

We’d all like to envision the planets as being spheres. Mathematically, spheres are much easier to model. Also, we have inherited a “perfection bias” from the ancient Greeks who described the heavens as a pristine sphere which contains only spheres.

We can understand why such a belief would have developed, for at initial glance, the planets which move across the firmament appear spherical. It was only when precise measurement techniques were developed that the truth about planetary shapes became known.

LEAVE A REPLY

Please enter your comment!
Please enter your name here