GIBBOUS TO FULL MOON

The Moon will be full on Oct. 2. When full, the Moon rises around sunset and will remain in the sky all night.

The Moon will be in the waxing gibbous phase on Monday and then will be in the waning gibbous phase from Wednesday until Oct. 10, when it will be at the third quarter point.

WHY IS VENUS SO BRIGHT?

A couple of weeks ago, we directed your attention to the early morning sky, where our sister world Venus currently resides. Venus outshines every planet and night-sky star. If you’re observing the eastern pre-dawn sky during the next few weeks, you’ll have no trouble finding this brilliant planet at all.

Have you ever wondered what makes Venus so bright?

Or, more correctly, why does it APPEAR to be so bright?

We attribute Venus’ brightness to four different factors: (1) Its close proximity to the Sun; (2) The amount of sunlight it reflects; (3) Its size and (4) Its close proximity to Earth.

Venus is the second planet from the Sun. It is, on average, about 26 million miles closer to the Sun than Earth. (Venus’ mean distance is 67 million miles. Compare to Earth’s mean distance of 93 million miles.) A planet’s distance from the Sun determines the intensity of sunlight that planet receives. Light intensity diminishes with the square of the distance, so those 26 million miles make quite a difference. Therefore, the sunlight Venus receives is more intense than that which strikes our planet.

A planet will not just absorb all the radiant energy imparted to it by the Sun. Some of this energy will be reflected back out into space. The ratio of the sunlight reflected to the sunlight received is known as the planet’s “albedo.” Venus’ albedo is nearly seven, meaning that it reflects nearly 70 percent of all incidental sunlight back out into space. This high albedo is a consequence of the planet’s pervasive cloud cover. These clouds tend to reflect sunlight quite efficiently.

Now, Venus is almost the same size as Earth. Each square meter of Venus’ cloudtops reflect much more solar energy than a square meter of Earth’s surface. (Earth’s albedo is 0.33) Now, if both planets have approximately the same area over which to reflect sunlight, then Venus will certainly be the superior reflector.

In reference to the first factor, distance makes all the difference when it comes to light. Venus is the closest planet to Earth. Thus, we receive more light from Venus than somebody on Mars would, for instance. (Not that there’s actually anybody on Mars.)

So, by taking all these factors into account, one can well understand why Venus’s appearance in our sky is so noticeable.

CASSIOPEIA: QUEEN OF THE CIRCUMPOLARS

Not every star in our sky sets. There are a certain number of stars, called circumpolars, that will not set (At least they won’t in our lifetimes.)

Skywatchers can see these patterns at anytime of night, and at anytime of year. The most famous circumpolar constellation for us northern-latitude sky watchers is probably the Big Dipper. This seven-star constellation looks like a soup ladle, with four stars marking the bowl and three stars representing a crooked handle.

This evening, you’ll find the Big Dipper low in the northwestern sky.

In the northeastern sky this evening, you’ll see the w-shaped Cassiopeia the Queen. Cassiopeia is another circumpolar. If you’re not out this evening, you can find Cassiopeia high in the northeastern sky by midnight. Mythologically, Cassiopeia was an Ancient Ethiopian Queen.

Her husband, Cepheus the King, resides in the same region of the sky as Cassiopeia. Cepheus consists primarily of faint stars shaped like a child’s drawing of a house: one square with a triangle for a “roof.”

The circumpolars are the comforting constants of the sky. You can see them as easily now as you can anytime of year.

For information about planetarium show times, please call 780-4249.

We offer public shows on Friday and Saturday evenings at 7 and 8:30 p.m. We also have 3 p.m. matinees on Saturday and will start Sunday matinees in October.

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