Discovering the Universe for Yourself

Flashcards covering patterns in the night sky, the reason for seasons, the moon and planetary motion.

What is a constellation?

A region of the sky, with eighty-eight filling the entire sky.

What is the celestial sphere?

Stars at different distances all appear to lie on it, and it is covered by the 88 official constellations.

What is the meridian?

line passing through zenith and connecting N and S points on horizon

What is the Zenith?

The point directly overhead

What is the Horizon?

All points 90 degrees away from zenith

Why do stars appear to circle from east to west?

Earth rotates from west to east.

Why do the constellations we see depend on latitude and time of year?

Your position on Earth determines which constellations remain below the horizon, and Earth's orbit changes the apparent location of the Sun among the stars.

What is latitude?

Position north or south of equator.

What is longitude?

Position east or west of prime meridian (runs through Greenwich, England).

What causes the seasons?

The tilt of the Earth's axis causes sunlight to hit different parts of the Earth more directly during the summer and less directly during the winter.

What is the Northern summer (June) solstice?

highest path; rise and set at most extreme north of due east

What is the Northern winter (December) solstice?

lowest path; rise and set at most extreme south of due east

What happens during equinoxes?

Sun rises precisely due east and sets precisely due west.

How does the orientation of Earth's axis change with time?

The tilt remains about 23.5 degrees, but Earth has a 26,000 year precession cycle that slowly and subtly changes the orientation of Earth's axis.

Why do we see phases of the Moon?

Lunar phases are a consequence of the Moon's orbit around Earth.

What is synchronous rotation?

The Moon rotates exactly once with each orbit, so only one side is visible from Earth.

What is a lunar eclipse?

Earth's shadow on the Moon

What is a solar eclipse?

Moon's shadow on Earth

When can lunar eclipses occur?

Lunar eclipses can occur only at full moon.

When can solar eclipses occur?

Solar eclipses can occur only at new moon

Why don’t we have an eclipse at every New and Full Moon?

The Moon's orbit is tilted 5 degrees to ecliptic plane.

What are the planets known in ancient times?

Mercury, Venus, Mars, Jupiter, Saturn

Why was planetary motion so hard to explain?

Planets usually move slightly eastward from night to night relative to the stars, but sometimes they go westward relative to the stars for a few weeks.

What causes apparent retrograde motion?

Occurs when we "lap" another planet (or when Mercury or Venus laps us).

Why Did the Ancient Greeks Reject the Real Explanation for Planetary Motion?

They did not think the stars could be that far away, thus the stage was set for the long, historical showdown between Earth-centered and Sun-centered systems.