Unit 1 : Activity 1 : Sunrise, Sunset, Star-rise, Star-set : Crescent Venus : The Wanderers in our Sky : Unit Exam









Sunrise, Sunset, & Star-rise, Star-set

The position of the sunrise shifts along the horizon over the course of a year. The position of sunset also shifts by the same amount. How far this migration of the sunrise or sunset extends along the horizon depends on where you live. At the equator, the sunrise position migrates plus or minus 23.5° from due East, while at Minneapolis, which is located 45°N latitude, the position can vary about plus or minus 35° from due East. At the arctic circle the change is plus or minus 90° which is the maximum possible amount.

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Ancient astronomers could easily mark the extreme positions of sunrise and sunset. All they had to do was stand in the same spot every day (a fixed observing position) and use large stones, trees, or other landmarks to mark the point in their view where the sun rose or set. These observations of the sunrise position could then be used to set up a calendar for the year.

In contrast, the rising and setting positions of stars don't change over the course of a year, but the rising and setting times do change. Using a fairly accurate clock you could observe that a given star sets (and rises) 4 minutes earlier per day. The average digital wristwatch more than qualifies to use for measurements in this case.

If you add this time up over the course of a month is comes to about 2 hours earlier per month, or 24 hours per year. So if a star rises at 3AM today, what time will it rise one year from today? Answer: It will rise at the same time on the same date every year (give or take a day depending on where we are in the leap year cycle).

A result of this gradual change in rising and setting times is that different constellations are up in the evening at different times of the year. Orion, Taurus, Gemini, and Canis Major and Minor are high in the sky to the South in the winter evenings as seen from North America or Europe. In the summer evenings, Scorpius and Sagittarius are most obvious in the southern sky. The "summer triangle" is very high in the sky for these same locations during June, July and August evenings.

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Another ancient calendar marker was the first time that a certain star could be seen in the glow of morning twilight, rising ahead of the Sun. This phenomenon was called "heliacal rising".

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First, the Sun rises ahead of the star. Then, the star is lost in the glow of sunrise. Finally, the star is visible ahead of the sunrise, and we have our marker for the calendar.

The moon and the planets, on the other hand, have more complicated apparent motions in the sky. As you may have noticed in Activity One, Mars scoots around the sky fairly quickly, passing through one constellation of the zodiac every two months, and showing up in the evening sky about once every two years. In contrast, Neptune and Pluto, move very slowly with respect to the stars. Pluto, for instance, shifts by less than 2° per year against the background stars. This makes sense, because Mars is much closer than Pluto and the motion of the closer planet is easier to detect. In addition, planets farther from the Sun move more slowly. Pluto, for example, goes about 40 times as far in its trip around the Sun as we do. It takes Pluto 250 years to travel once around, so its average speed is 40/250 or 16% of Earth's speed around the Sun.

The Moon’s motion changes the fastest of any body we can see. Once about every 29 days the Moon appears as a thin crescent in the evening sky just after Sunset and sets shortly after the Sun sets. Later in the month, the moon appears to slowly grow fatter and sets later and later in the evening. Eventually, the moon rises at sunset and is full in the sky all night. The moon again rises later and later, the lighted portion becomes thinner. Finally, the moon is a thin crescent rising just before the Sun rises. Soon after that, the cycles starts over again.

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If you observe Venus in the evening sky, you may notice that it appears higher and higher at sunset for a while. Then, it will reverse direction and be lower and lower until you can't see it any more in the glare of sunset. If you then start watching for it in the morning sky, you'll see it higher and higher at sunrise, until it again reverses. This whole cycle, the synodic period of Venus, takes about 584 days or 1.6 years. Mercury has a similar pattern, although it is harder to see because it never gets very far from the Sun. Mercury also has a shorter synodic period. The other planets can appear any distance from the Sun along the ecliptic, but Venus and Mercury stay close to the Sun. Why?