Astronomy 100
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Stars' brightnesses are given either in units of intensity (energy received per second per unit area on the Earth's surface) or, more commonly, in magnitudes, which come from the Greek astronomer Hipparchus.
Magnitudes go backwards! And they're logarithmic (small number difference = big difference in light). Smaller (or negative) magnitudes are bright, larger (more positive) magnitudes are faint. The brightest nighttime star is Sirius, at magnitude -1.47. The Sun has magnitude -27. The faintest stars visible to the naked eye are around 6th magnitude.
A difference of 5 magnitudes gives a difference of 100 times in intensity. Every one magnitude step is a difference of 2.512 times in intensity.
Intensity changes 100x for every 5 magnitudes, and there are 5x5 magnitudes difference. 100x100x100x100x100 = (102)5 = 1010 = 10 billion!
This tilt makes the ecliptic (and the zodiac) appear to be tilted with respect to the celestial equator, so during summer (June to September) the Sun appears north of the celestial equator -- and high in the sky for us Northerners -- and during winter (December to March) -- the Sun appears south of the celestial equator -- and low in the sky for us.
Check this with the length of your shadow at noon in summer vs. winter!
(But didn't you say the Earth's orbit was elliptical, so sometimes it's closer to the Sun, and sometimes farther? Why doesn't THAT cause the seasons?)
The equinoxes correspond to positions in the Earth's orbit when the Earth's axis is pointing neither towards nor away from the Sun, but "sideways" to it.
On the equinoxes, the Sun rises due east and sets due west. All the rest of the year it rises and sets either north of east/west, or south of east/west.
When the Earth's orbital revolution brings it around to a position in which the north pole of its axis points towards the Sun, we see the Sun achieve its farthest north position in the sky in the whole year. The sun appears to hang in that position for several days, moving neither higher nor lower (of course it still rises and sets every day, and still slips from west to east with respect to the stars). This is the summer solstice (sol = Sun, stice = stands still). Likewise, when Earth's northern axis points away from the Sun, the Sun reaches its lowest (southernmost) position in the sky, and we have the winter solstice.
The apparent height of the Sun (or any celestial object) above the horizon at any given time and place is called its altitude, and it's measured in degrees.
Example: What is the altitude of the Sun at noon on March 21, as seen from Amherst (latitude 42o north of the equator)? If we were at the north pole, the Sun would appear to lie on the horizon (altitude = 0o). For every degree of latitude we march from the north pole towards the equator, the sun would creep one degree higher in the sky. By the time we got to Amherst, after walking 90-42 = 48o south, the Sun would appear at an altitude 48o above the horizon.
As the Earth's axis precesses, it sweeps out a huge, slow circle in the sky, pointing now very close to the star Polaris, but sometimes pointing to other stars. Our North Star was not the North Star of the ancient Egyptians!
Precession also causes the positions of the equinoxes to creep slowly around the zodiac, so the Sun appears in different constellations on a given date now than it did thousands of years ago, when astrology and astronomy were one and the same. For instance, if your birthday is today (Feb. 3), your "sun sign" is Aquarius. On Feb. 3, 3000 BC, the Sun would have appeared in the constellation Aquarius. But today, the Sun appears in Capricorn on Feb. 3, one constellation over from Aquarius. So it goes for the rest of the signs of the zodiac, and the rest of the year.
As the Moon orbits the Earth once a month, the phases of the Moon are caused by the light from the Sun coming from DIFFERENT DIRECTIONS (backlit, sidelit, frontlit) throughout the month.
Try this: Hold an egg (the Moon) in font of you (the Earth) and face a very bright light (the Sun) in an otherwise darkened room. Now continue to hold the egg in front of your face as you slowly spin counterclockwise. What happens to the shape of the light and dark sides of the Moon-egg?
Note: The phases of the Moon have NOTHING to do with shadows from the Earth, or clouds, or anything else (apart from the Moon itself) "getting in the way" of the light from the Sun.
The Moon's synodic period (time till it lines up with the Earth-Sun line again) is the same as its rotation period, 29.53 days. This means we always see the same side of the Moon , whether it's dark, light, or only partly lit. (There's a little bit of wobble in the Moon's rotation that lets us see around the sides, but not much.) It wasn't until we sent artificial satellites to the Moon in the 1960's that we first saw the back side of the Moon.
The next time you look at the Moon, ask yourself where the Sun must be
in order to illuminate the Moon that way.
When the Moon blocks out part or all of the photosphere (bright part) of the Sun as seen from Earth, we call it a partial or total solar eclipse . We're incredibly lucky to be able to see this at all: The Moon is 400 times smaller than the Sun, but it's also 400 times closer to Earth (pure coincidence!), so they look almost exactly the same size.
Why would the Moon and Sun appear to change size at all? Because the Moon's and Earth's orbits are elliptical, not circular, so sometimes the Moon is close to earth (perigee -- Moon looks big) while the Earth is far from the Sun (aphelion -- Sun looks small), or vice versa (apogee and perihelion)
During total solar eclipses, the umbra (dark part) of the Moon's shadow touches part of the Earth. During partial solar eclipses, only the penumbra (light part) of the Moon's shadow touches the Earth.
During total solar eclipses, you can see the Sun's corona (glowing hot halo), chromosphere (the layer just above the photosphere), and prominences (mountains of red gas erupting from the surface) -- all usually invisible in the glare of the photosphere.
Total solar eclipses happen only at New Moon (Moon is in between
Earth and Sun, so we see black, unlit part = New Moon).
Now switch the Earth and the Moon, and you get a Lunar Eclipse , when the Earth blocks out the Sun, as seen from the Moon. What do we see on Earth? A dark Moon!
If the Moon passes too low or too high in its orbit, there will be a partial lunar eclipse, or no eclipse at all.
Some sunlight still gets through to the Moon, after passing through the Earth's atmosphere and getting bent towards the Moon; this usually makes the Moon appear red or coppery.
Lunar eclipses occur only at Full
Moon (Earth is between Moon and Sun, so we see bright, fully-lit part
= Full Moon).
For an eclipse of either type to occur, the Moon must be close to the line of nodes (the line where the Moon's orbit around the Earth and the Earth's orbit around the Sun intersect). What happens if it's NOT near the line of nodes?
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