Ch.2 Naked Eye Universe

Celestial sphere

Imaginary sphere used to map positions of objects in the sky.

Constellation

Recognizable pattern or defined region of stars in the sky.

Why are constellations useful?

They provide a reference background for tracking sky motions.

Daily apparent sky motion

Sun, Moon, and stars generally rise east and set west because Earth rotates eastward.

Celestial poles

Points in the sky aligned with Earth's rotation axis.

Circumpolar stars

Stars that never set at a given observing latitude.

Ecliptic

Apparent yearly path of the Sun against background stars.

Why does the Sun drift eastward against stars?

Earth revolves around the Sun.

Moon's motion against stars

Moon gradually moves eastward relative to background stars.

Moon phases

Changing appearance of Moon's sunlit half as seen from Earth.

Cause of Moon phases

Changing Sun–Earth–Moon viewing geometry, not Earth's shadow.

New Moon

Moon lies roughly toward the Sun; illuminated half faces away from Earth.

Full Moon

Earth-facing lunar hemisphere appears illuminated.

First-quarter Moon

Half of the visible Moon is lit while waxing.

Third-quarter Moon

Half of the visible Moon is lit while waning.

Waxing

Illuminated fraction of the Moon appears to increase.

Waning

Illuminated fraction of the Moon appears to decrease.

Solar eclipse

Moon passes between Earth and Sun and casts a shadow on Earth.

Lunar eclipse

Earth passes between Sun and Moon, shadowing the Moon.

Umbra

Region of complete shadow.

Penumbra

Region of partial shadow.

Why not an eclipse every month?

Moon's orbital plane is tilted relative to Earth's orbit.

Why can the Moon cover the Sun?

Their apparent angular sizes are nearly equal.

Annular solar eclipse

Moon appears too small to cover Sun fully, leaving a bright ring.

Cause of seasons

Earth's tilted axis changes daylight duration and Sun angle.

Why is summer warmer?

Longer days and more direct sunlight deliver more energy.

Are seasons caused by Earth–Sun distance?

No; axial tilt is the main cause.

June solstice in Northern Hemisphere

Sun follows its highest yearly path; longest daylight.

December solstice in Northern Hemisphere

Sun follows its lowest yearly path; shortest daylight.

Equinox

Sun rises due east and sets due west; day and night nearly equal.

Why do seasonal changes grow at high latitudes?

Tilt produces larger differences in daylight and solar angle.

At midnight where are meridian stars relative to Sun?

On the opposite side of the sky from the Sun.

Why are some constellations not visible at night in a season?

They lie near the Sun in the daytime sky.