01 The Night Sky and Its Motions Free Online FlashCards

What is the celestial sphere?

The celestial sphere is an imaginary sphere surrounding Earth, used to map the apparent directions of celestial objects.

What is a constellation?

A constellation is an officially defined region of the sky; its familiar star pattern is a useful landmark, not necessarily a group of nearby stars.

What causes the sky’s daily apparent motion?

Earth rotates west to east, making celestial objects appear to move from east to west across the sky each day.

Why do evening constellations change through the year?

As Earth orbits the Sun, nighttime faces different parts of space, so the constellations visible in the evening shift with the seasons.

What is the ecliptic?

The ecliptic is the Sun’s apparent yearly path across the celestial sphere.

What primarily causes Earth’s seasons?

Earth’s axial tilt of about $23.5^\circ$ makes each hemisphere alternately tilt toward and away from the Sun, changing sunlight angle and daylight length.

What happens at a solstice?

At a solstice, a hemisphere is tilted most strongly toward or away from the Sun, marking its longest or shortest day of the year.

What happens at an equinox?

At an equinox, neither hemisphere tilts toward the Sun more than the other, and day and night are approximately equal in length.

Why does the Moon have phases?

The Moon’s phases occur because sunlight illuminates half the Moon, and as the Moon orbits Earth, we see different portions of that sunlit half.

What do “waxing” and “waning” mean for the Moon?

Waxing means the visible lit portion is growing; waning means it is shrinking.

How long is the cycle from new Moon to new Moon?

The cycle from one new Moon to the next takes about $29.5$ days.

How does a solar eclipse occur?

A solar eclipse occurs at new Moon when the Moon passes between Earth and the Sun, casting its shadow on part of Earth.