Moon watch

Even though we’re concentrating on the planets, don’t forget to observe the Moon this week as it passes through the waxing gibbous phase. The Moon was full on Saturday, April 27.

Planet watch

For the first time in decades, you can still see all the naked-eye planets (Mercury, Venus, Mars, Jupiter and Saturn) in the western evening sky.

Mercury emerged into our evening sky on April 18, 2002. Watch this fast moving planet rising higher each night in the western evening sky for the next couple of weeks.

Venus is low in the southwestern evening sky. Venus sets about two hours after the Sun.

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.

Planets in a row?

Anytime the planets are aligned in any special way, the press takes notice. Consequently, many people inquire about the nature of planetary alignments. Quite often during a special alignment, someone will ask if the planets are arranged in a neat row with respect to the Sun.

The answer is always “No.” Why? Well, because the planets can NEVER all be in a perfect row with respect to the Sun.

Let’s start with a definition: When two planets are aligned perfectly with respect to our Sun, they are said to be in “heliocentric conjunction.” (Heliocentric meaning “sun-centered,” after the Greek God Helios.)

For instance, when Jupiter is in opposition, Earth is between that planet and the Sun. If you were standing on a platform above the Sun, you’d see Earth and Jupiter come into a line. At the same time you’d see the other seven planets arranged in various stations around their respective orbits.

Now, how long would you have to stand on that platform to watch all the planets move into heliocentric conjunction simultaneously?

Well, you’d basically stand there forever, considering that even three planets will almost never be in heliocentric conjunction at the same time.

You see, heliocentric conjunctions are instantaneous.

Planets are always moving and at variable speeds. Thus, Earth and Jupiter spend just an instant in heliocentric conjunction before the faster moving Earth passes by it. It would be nearly impossible for any other world to enter into heliocentric conjunction at that exact time.

So, one can safely assume that we’ll never see all the planets in a neat row.

Just for purposes of argument, let’s assume that we arranged the planets in this neat row and then let them loose in their orbits.

Mercury and Venus will whip along quickly, while the outer planets lumber along. This motion variation is a consequence of the fact that the closer a planet is to the Sun, the greater its orbital velocity will be.

How long would it take for them to come back into line?

Well, it is possible that they’d return to their original alignment in 12,000,000,000,000,000,000,000 years. No guarantees, but it is possible.

That span is about a trillion years longer than the age of the Universe! Quite a long time to wait on a platform, isn’t it?

And, in fact, you can infer from this exercise that no planetary arrangement is ever repeated exactly. The configuration of the planets tonight (Mercury to Pluto) is unique.

So, in fact, every night we have an arrangement of worlds we’ll never see again.

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