Sunday, March 4, 2012


Susan Arastradero's photo of a Moon-Venus conjunction.

Q0. If there is no reflection of the photograph, and assuming the photo is taken in the northern heimsphere, can you tell whether it is at sunrise or sunset?

Answer: First determine the ecliptic, this is the plane containing the observer and the "arrow" defined by the Moon's "bow". IN the northern hemisphere, the zenith of the ecliptic is in the south, so S is to your right, North to your left and hence you are facing East and it is sunrise. Susan had mentioned to me that she had not traveled outside CA to anywhere but Arizona and Cuba, which are all in the northern hemisphere. Then she mentions in her e-mail that she had just enough time to take the picture before the sky became too light.

Here is another puzzle, based on an excerpt from her e-mail: "At first I thought the shape of Venus was blurred 'till a friend pointed out that it was earth's shadow."

Q1. What do you discern of Venus' shape?

Answer: A crescent.

Q2. Why is it impossible that the crescent is defined by earth's shadow?

Answer: I'll leave that to you.

Q3. What is the cause of the crescent shape of Venus?

Answer: That is the part of Venus that is lit by the Sun! The gedankenarrow in the bow formed by Venus' crescent points towards the Sun, as does the crescent Moon.

Q4. Is Venus on the near or the far side of the Sun?

Answer: The near side. If it were at the points on its orbit tangent to us, i.e. the Sun and Venus were to be equidistant to us, we would see Venus exactly half lit. If it were at points further, and hence on the other side, we would see more of its face lit (in extremis in opposition to us or in far conjunction with the Sun we would see the Full Venus) and hence a gibbous Venus. Since we see a crescent Venus, we know it is on the near side, since we see less than half of its face lit by the Sun (again, in extremis, when Venus is in near conjunction with the Sun we see a "dark" or New Venus).

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