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Assignment 22-5

Angles on the sky:

Often in war movies you will hear the pilot of an airplane say, “enemies at 3 o’clock.” What does that mean?

Are they saying that the enemy will be coming at 3 o’clock in the afternoon? No, of course not. Pilots and astronomers use what is called declination and right ascension to talk about different parts of the sky.

Right Ascension (abbreviated RA) and Declination (abbreviated DEC) are a system of coordinates used by astronomers to keep track of where stars and galaxies are in the sky. They are similar to the system of “longitude” and “latitude” used on the Earth.

Declination is measured in degrees and refers to how far above the imaginary “celestial equator” an object is (like latitude on the Earth). Try standing in the middle of a room and hold your arm out straight in front of you. If you move your arm up to point at a light or the ceiling, it is just like going “up” in Declination. If you move your arm down to point at some objects on the floor, you’re moving “down” in Declination.

Declination, like latitude, is measured as 0 degrees at the equator, +90 degrees at the North Pole, and -90 degrees at the South Pole.

Right Ascension measures the other part of a star’s position. It is similar to longitude on the Earth. As you are standing in the room, if you spin yourself clockwise to point at a door, then a window, then another door, you are “moving” in Right Ascension.

Right Ascension is measured in hours of time. This is convenient for astronomers because as the Earth rotates, stars appear to rise and set just like the Sun. If you go out into your backyard in the winter and lie on your back some night, you might be able to see the constellation of Orion overhead. Orion has a Right Ascension of 5 hours. Out of the corner of your eye you might also see the constellation Cancer, which is at a Right Ascension of 8 hours. This means that if you wait 3 hours (subtract 5 hours from 8 hours), Cancer will be directly overhead.

Just as latitude and longitude uniquely identify the positions of cities on the Earth, Right Ascension and Declination uniquely identify the position of stars and galaxies in the sky.

Knowing how to use right ascension and declination is very important in Astronomy.

The Ecliptic:

Another very important part of the sky is called the Ecliptic. To understand what this is we need to talk a little more about our Solar System. Remember that the Solar System is shaped like a disk, kind of like a big flat wheel. All the planets circle the Sun on that disk.

In our sky we can see five of the planets. Because they are all on a disk, the planets appear to cross our sky in a line or trail. That trail where the planets cross is called the Ecliptic. The Sun and Moon also follow the ecliptic.

Another name for the Ecliptic is Zodiac. This trail crosses through 13 constellations. As the Sun travels across the trail it passes through each of these 13 constellations. Which ever constellation the Sun is in when you are born is what sign you are said to be. I was born in May. In May the Sun is in the constellation Taurus so my sign is Taurus.


Assignment # 5

Search the word “Zodiac” on the Internet. List 5 of its constellations.


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Any five of the following constellations are correct.

  • Aquarius
  • Gemini
  • Libra
  • Pisces
  • Cancer
  • Scorpio
  • Aries
  • Leo
  • Sagittarius
  • Taurus
  • Virgo
  • Capricorn