Notes 1-3: Observing the sky with the naked eye 1/9/09

Observing with the naked eye refers to observing the sky without the aide of a telescope or binoculars. There is a lot to see with the naked eye and becoming proficient at observing the sky with the naked eye is a good idea before trying to locate objects through a scope. Obviously, using a scope allows us to see objects that are too small or too faint to observe with the naked eye, but there is still a lot we can observe without it. Items we can observe in the sky include The Sun (Note: NEVER look directly at the sun) The moon 5 planets (Mercury, Venus, Mars, Jupiter and Saturn) Comets Stars and constellations The Milky Way

Unfortunately, we live in a city with a lot of light pollution. Light pollution happens when the bright lights of a city shine upward into the sky. This lights up the atmosphere and makes fainter stars hard to see. If you have never had the opportunity to do so, I suggest you find time to get outside of the city and see the night sky, it is truly spectacular how many stars can be seen.

Star Brightness Stars and other objects visible in the sky are categorized by their brightness. Each object is given a brightness rating on a scale called the apparent magnitude scale. On this scale, the brighter the object, the smaller the number. Very faint objects in the sky have a larger apparent magnitude.

Star Brightness As you can see, the brightest objects in our sky have negative magnitudes. The sun is the brightest object in the sky with an apparent magnitude of –26.5. The faintest objects we can see with our naked eye have a magnitude of about 6. To see fainter objects in the sky we must use a telescope.

Our night Sky What constellation is shown in the picture above? If you do not know, can you name any one constellation?

Constellations A pattern of stars in the sky, usually named after mythological God’s, people, animals or objects.

Constellations The constellation you see above is one of the more recognizable constellations in the northern hemisphere. This is Orion the Hunter. Do you see a hunter?

Constellations The constellation you see above is one of the more recognizable constellations in the northern hemisphere. This is Orion the Hunter. Do you see a hunter?

Constellations Stars were grouped into constellations by different cultures as far back as ancient cultures. Why?

Constellations Stars were grouped into constellations by different cultures as far back as ancient cultures. Why? Ancient cultures likely grouped stars into constellations to help them remember the stars in the sky. The first people to do this were farmers. Certain constellations were visible only in the spring and others in fall, these constellations being visible told the farmers when to plant and when to reap.

Constellations Stars were grouped into constellations by different cultures as far back as ancient cultures. Why? Ancient cultures likely grouped stars into constellations to help them remember the stars in the sky. The first people to do this were farmers. Certain constellations were visible only in the spring and others in fall, these constellations being visible told the farmers when to plant and when to reap. Constellations have also been used for navigation on land and sea.

Constellations As you can imagine, different cultures came up with their own constellations and their own names for them. There was not uniformity from one culture to the next.

Constellations As you can imagine, different cultures came up with their own constellations and their own names for them. There was not uniformity from one culture to the next. In 1928 the International Astronomical Union established 88 official constellations with clearly defined boundaries. Now, a constellation not only marked a group of stars but a region in the sky and all the stars and objects in that region belonged to that constellation. There was no overlap and no stars left out.

Constellations Due to the spin of the Earth on its axis, the stars move through the sky throughout the night. The celestial sphere spins one full time each day (actually, the Earth does). Due to the motion of the Earth around the sun, the stars that are visible at night vary from season to season. Some stars are actually visible every night of the year and at all times, we call these the circumpolar stars. The north star is an example of a circumpolar star.

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Circumpolar Stars

You are here Circumpolar Stars Southern stars that cannot be seen from our location

To find certain stars and other objects in the sky there are a few systems used. Using celestial coordinates is one way to pinpoint the location of a star. Celestial coordinates are similar to use latitude and longitude coordinates here on Earth. If you could imagine the grid system of the latitude and longitude lines on Earth extended outward to the celestial sphere you could picture the celestial coordinate system. Celestial “latitude” is called the declination. Declination is measured in degrees north and south of the celestial equator. It tells us how high in the sky an object is. The celestial equator is 0°. North celestial pole is at 90° and the south celestial pole is at -90°.

Celestial “longitude” is called the right ascension. Right ascension is a little more difficult to measure than declination because the location of zero is harder to pinpoint. Zero right ascension is marked by the point on the celestial equator where the vernal (spring) equinox occurs. Instead of being measured in degrees, right ascension is measured in hours, minutes and second. This mirrors the measurement of time on purpose, because there are 24 hours of right ascension around the world and 24 hours of time in one day.

Another way to locate objects in the sky is to explain which constellation they can be found in. The major stars of a constellation are ranked in order of brightness and then named for this. For example, if you are looking for the star Rigel you can find that it is also known as Orionis β. This tells us the it is located in the constellation Orion and is the second brightest star (because β is the second letter of the Greek alphabet).

If I tell you that tonight the planet Saturn can be located in the constellation of Leo, would you know where to look to find the planet? The planet would be easy to locate if you knew where the constellation Leo was located and what it looked like. To familiarize ourselves with the constellations and stars, we are going to learn to use a Star Finder, like the one seen below. This will become an invaluable tool for observing the night sky.