Understanding The Sky Astronomy is full of cycles Like the day, the month, & the year In this section we will try to understand these cycles. For Example Why do we think of stars as nighttime objects? What happens to them during the day? 1
Why is the Sky Blue? Violet light is scattered 10 times more than red light Blue light is scattered 7 times more than red light Green light is scattered 5 times more than red light Yll Yellow lihi light is scattered d3i times more than red light Orange light is scattered 2 times more than red light So Why is the Sky Red Sometimes? Sometimes we see the reddish colors because the blue light has already been scattered out and away from the line of sight 2
Is this a picture of Night or Day on the Moon? Earth s Atmosphere The color of the sky actually gets darker and darker the higher up you go Indicating that there is less and less scattering ggoing gon! 3
Star Charts While most star maps look flat, we must actually consider the Earth in the middle of a 3-dimensional forest of stars; a great, hollow, sphere that surrounds the Earth Only the brightest stars actually have names 4
Constellations Many groups of stars are combined together to form constellations Ancient astronomers saw almost exactly the same patterns of stars thousands of years ago that we do now. They called these patterns constellations There are only 88 officially named constellations which section off the night sky similar to the way the United States is divided into states. 5
More with Orion The Hunter Note that the stars in a constellation not necessarily at the same distance from the Earth, and are probably quite unrelated to each other in space. We measure both distances and sizes using angles since we can t make these measurements directly Angles 6
Think of a circle How many degrees are there all the way around? Each one of those can be divided in to 60 parts Each one of those can also be divided into 60 parts This gives a total of 3600 sections that one degree can be broken down into... or... 1,296,000 sections a complete circle can be divided into!!! By sighting from the top and bottom of an object its angular diameter can be found This relates directly to the actual diameter (size) if the distance to that object is known. Both the Moon and the Sun are about ½ of a degree across. 7
Measuring Some Angles You ll note that the entire sky of stars appears to move across the sky over a single night Even though the pattern of stars remains unchanged at least in our lifetime!! 8
Case Study: The Big Dipper The motion of the stars Today 50,000 BC Today 50,000 AD 9
The effect is now known to be caused by the rotation of the Earth under the 3-D forest of stars How do we know the Earth rotates? Foucault's Pendulum 10
The Celestial Sphere This 3-D forest of stars is called the Celestial Sphere. Other points of interest 11
The daily rotation of the Earth as viewed from above the north pole is counterclockwise This gives the view that the stars and the Sun rise in the East and Set in the West Locating Objects on Earth and in Space Latitude Our position relative to the Equator. Measured North (+90 o ) or South (-90 o ) Pikeville is about +38 o North Latitude 12
Declination Position of the stars on the celestial sphere relative to the celestial equator Measured North (+90 o ) or South (-90 o ) Your declination determines which constellations you see 13
And for how long... What would the motion of the stars look like at the poles? What about at the equator? 14
Longitude Our position relative to the Prime Meridian Measured East (+180 o ) or West (-180 o ) Pikeville is about -83 o West longitude Right Ascension Position of the stars on the celestial sphere relative to the Vernal Equinox Measured around the celestial equator Measured in HOURS!!!! 15
So Right Ascension is more about TIME than location! For Example... Go outside in the winter and look straight ahead at the constellation Orion. 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, without moving your head, if you wait 3 hours (subtract 5 hours from 8 hours), Cancer will be directly in front of you. 16
So to address a star you need both its declination and right ascension Time Time has always been important! Historically our measurement of time was always based on the rotation ti of the Earth. These ancient people figured out that it took about 365 days to rotate through the four seasons. Really more like 365.25 days If we didn't have leap years we'd get six hours behind every yyear. It doesn't sound like a lot, but over time, it adds up. In reality it actually takes Earth a tiny bit less than 365.25 days to travel around the Sun (365.242199, if you're keeping score). 17
Motion Around The Sun Because the Earth also goes around the Sun the constellations we see throughout the year change Summer Winter 18
Ecliptic Path the Sun follows across the sky The RED line is the Celestial Equator The GREEN line is the Ecliptic As the sun follows this path, it passes over twelve of the 88 constellations; those called the Zodiac. 19
The direction the Earth goes around the Sun is the same counterclockwise direction as the Earth spins We ll see later on that the Sun also rotates CCW as well as do the other planets in the Solar System This information will be used later when we talk This information will be used later when we talk about the formation of the Sun and planets 20
The Earth spins about 361 degrees around it axis in order for the Sun and all the other stars to be in the same position in the sky. The extra one degree per day adds up to one whole day; 366 rotations instead of 365. Another way of looking at this is that the Earth rotates once in 23h 56 minutes. This is called a sidereal day and is very important for astronomers. Leap years arise every 4 years because the Earth does not rotate a whole number of times in one year. The Seasons The Earth is tilted 23.5 o in space resulting in half the Earth getting more direct sunlight for part of the year compared to the other half of the Earth. 21
The Tilt Zooming in... Sunlight shining more direct makes you feel warmer 22
As we noted earlier, the tilt makes the Ecliptic different from Celestial Equator Some added affects While the northern hemisphere is experiencing summer the southern hemisphere is experiencing winter Some locations very near the poles don t see the Sun rise/set for 6 months!! The Earth is actually closer and farther from the Sun during certain points of the year...does this matter too? 23
We also experience different lengths of daylight due to the tilt We re tilted in such a way that a star is nearly right above the North Pole of the Earth (at the North Celestial Pole). We call that star the North Star or Polaris. The North Star 24
The tilt stays at the same angle all the time but the top of the Earth sometimes points in different directions; i.e. NOT toward Polaris! We call this motion precession. Kind of like a wobbling top Precession A New North Star So the North Star changes with time! 25
The Phases of the Moon The changing appearance of the moon is caused by the changing angles of illumination from the Sun. When the illumination is mainly from the back we see a crescent moon. When it is mainly from the front we see a gibbous moon. Bits of information... As viewed from the Earth, the Moon takes about 28 days to orbit the Earth. The orbit is in the same counterclockwise direction as the rotation of the Earth and revolution of the Sun It is tilted by a few degrees from the plane of the Earth's orbit around the Sun. 26
A bit more... Moon keeps the same face (side) always pointing towards Earth. The only people who have seen the back of the Moon, in person, are astronauts. This concept is called locked lunar rotation and occurs as the Moon rotates on its own axis once every rotation around the Earth (about one month or 28 days). 27
Eclipses Eclipses occur when the Earth, Moon, and Sun are well enough aligned to form shadows on the Earth or the Moon. The Earth's diameter is 13,000,000 m (8000 miles). The Moon's diameter is about 1/4 of Earth's diameter. The Sun's diameter is about 108 x that of the Earth. The distance between the Moon and the Earth is about 30x Earth's diameter. The distance between the Earth and the Sun is about 375x Earth-Moon distance. These sizes account for the reason that both the Sun and Moon appear to be 1/2 o across. Lunar Eclipse A lunar eclipse occurs the Moon passes through the shadow of the Earth. A total lunar eclipse means that during this event no light from the Sun reaches the Moon. A partial lunar eclipse means that during this event that part of the Moon has no sunlight reaching it. A lunar eclipse is visible from half the world. A lunar eclipse can occur only during Full Moon. 28
We only get about 2 lunar eclipses a year since the Sun, Earth, and Moon don t always form a straight line A solar eclipse is a celestial event during which the New Moon passes directly between the Earth and Sun, temporarily blocking the Sun's light. A total solar eclipse occurs when all the Sun is blocked out. There are also partial ones Solar Eclipse The Chinese documented the first known solar eclipse in 709 B.C. 29
Totality occurs only on a very narrow path. Outside of this path you see a partial solar eclipse, which is much less spectacular. The distance from the Earth to the Moon varies causing the angular size of the Moon to change. Sometimes it easily blots out the Sun (when it is closer); other times it does not quite do so (when it is farther away). When the Moon fails to cover the whole disk of the Sun we get an annular eclipse. Annular Solar Eclipse 30
Eclipse Summary Frequency of Eclipses Eclipses do not occur every month because of the angle between Moon's orbit and plane of ecliptic. Like lunar, there are roughly two solar eclipses per year. 31
End Topic Understanding the Sky 32