Astronomy 100


Lectures Table of Contents Astro 100

Cycle of the Sky

The Seasons


  1. What Makes the Seasons?
  2. Equinoxes and Solstices

Terms to Know

equinoxes (Vernal and Autumnal)
solstices (Summer and Winter)

1. What Makes the Seasons?

The Earth's orbit is tilted at 23.5 degrees, like a top's, not straight up-and-down, perpendicular to the ecliptic. This means on one half of its annual orbit around the Sun, the North pole tips towards the Sun, and on the other half, the South pole tips towards the Sun. Tilting towards the Sun makes (1) the days longer and (2) the sunlight more intense, both of which cause that part of the Earth to heat up.

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?)

2. Equinoxes and Solstices

As the Sun appears to move along the ecliptic during the year, it crosses the celestial equator twice: once on its way north, and once on its way south. Those instants -- called the equinoxes -- when the Sun is exactly on the celestial equator correspond to the first day of spring (vernal equinox, around March 21) and the first day of fall (autumnal equinox, around Sept. 21). At those times, observers at the north and south poles of Earth see the Sun exactly on the horizon, and every place else on Earth sees 12 hours of day and 12 hours of night.

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.

Phases of the Moon; Tides; Eclipses


  1. Phases of the Moon
  2. Tides
  3. Eclipses

Terms to Know

Sidereal Period
Spring Tide
Neap Tide
Lunar Eclipse
Solar Eclipse

1. Phases of the Moon

The Sun shines by its own light, like a light bulb (a very bright one). The Moon, on the other hand, shines by REFLECTED light from the Sun. Without the Sun or some other source of light, the Moon would be completely black.

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.

2. Tides

The closer you are to something massive (like the Moon), the more strongly its gravitational force pulls on you. At any given moment, one point of the Earth is closer to the Moon than any other point, and the point on the opposite side of the Earth is farther than any other point. The difference in gravitational pull of the Moon on those points causes the watery, flexible Earth to stretch out like silly putty in a bulge towards and away from the Moon. As the Earth spins on its axis once a day, different part of its surface get carried through the bulge. This causes the tides: high water twice a day (towards and away from the Moon) and low water twice a day (in between the high tides). (Some local shorelines make things more complicated than this simplified case.)

3. Eclipses

Eclipses occur when something gets in the way of something else from a third party's perspective.

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?

Lectures Table of Contents Astro 100

Houjun Mo Astronomy 100