Summer, Winter, and the Tropics

The Sun will be overhead on the Tropic of Cancer on the summer solstice (Northern Hemisphere), and overhead at the Equator on the equinoxes. From a “pole-up” point of view, it shines down from the North during the summer, and up from the South in winter. The Poles are only illuminated one at a time.

The Reasons for Seasons

The Midnight SunAbove the Arctic Circle, the Sun never sets during summer. It travels a tilted circle around the sky (at the Pole the circle is flat).

Why Winter is Colder, even though the Sun is up…

Days are shorter, but the Sun is also less effective at heating the ground. That is because the sunlight is spread over a greater area, so a given area gets a smaller fraction of energy.

Just kidding….

Different Points of View

You can choose to make the Ecliptic “horizontal” or you can make the Equator “horizontal”. You could also replace the Sun with the Earth in this picture (going “geocentric”) and then it would be clear why the Sun shines “down” or “up” on either pole during the solstices.

Astro Quiz

Suppose the Earth’s rotation axis was perpendicular to its orbital plane (so the celestial equator was on the ecliptic). Which statement below would be FALSE?

1) There would not be any real seasons.

2) All stars would be circumpolar.

3) The length of the day would not vary by month or latitude.

What the seasons are NOT due to…

NOT the ellipticity of the Earth’s orbit: Our ellipticity is very small, and we are closest to the Sun in January.You can most easily see this by remembering that at the same time it is Winter here, it is Summer in the Southern Hemisphere.

NOT the fact that one pole is closer to the Sun because the Earth’saxis is tilted: The size of the Earth is miniscule compared with its distance to the Sun, so it doesn’t matter how things are oriented, they are all about the same distance from the Sun.

Note: Summer is not warmest at the solstice (June) because it takes a while for the seasonal effects to really kick in, so we tend to be hottest a couple of months later (and coldest a couple of months after the winter solstice).

Phases of the Moon

Where to find the Moon in the Sky

The angle between the Sun and the Moon determines its phase.

A crescent Moon must be near the Sun

A half Moon (first or last quarter) must be overhead when the Sun is rising or setting

The full Moon must rise when the Sun sets, and be on the meridian near midnight

The waning Moon will tend to be seen in the daytime

Phase Movie

Lunar EclipsesA lunar eclipse occurs when the Moon goes into the Earth’s shadow. Everyone who can see the Moon then sees the eclipse. It looks red because sunlight is refracted through our air.

The Moon must be full.

Solar EclipsesSolar eclipses happen when the Moon passes between us and the Sun. The eclipse can only be seen where the alignment is perfect. The sky goes dark, the stars come out, and the solar corona becomes visible. It only lasts a few minutes in any given spot. Although there are 1 or 2 per year, any given place has a long wait to see one.

The Moon must be new.

ShadowDance

Eclipses usually don’t happen, because the Moon’s orbit is tilted.

The Moon just happens to have about the same angular size as the Sun.This is a coincidence of the present, the distant past and future will not get perfect solar eclipses because the Moon’s distance is growing.

Astro Quiz

Under which condition below would there be a total solar eclipse every month?

1) The moon’s orbit crosses the ecliptic at 12 points.

2) The moon’s orbit appears to move along the ecliptic.

3) The moon’s orbit is perpendicular to the celestial equator.

Tides

Tides are caused by the difference in the pull of gravity from one body on the near and far side of another (1/r3). They are important in all kinds of astronomical contexts. They cause the Earth to slow down and the Moon to recede.