Chapter 1: The Night Sky

Roughly how many Earths would fit inside the Sun?

In 1 second, light leaving Los Angeles reaches approximately as far as

If Earth rotated twice as fast as it currently does, but its motion around the Sun stayed the same, then

When a thin crescent of the Moon is visible just before sunrise, the Moon is in its

If the Moon's orbit were a little longer, solar eclipses would be

Relative to the stars, as seen from Earth, through roughly how many degrees, arc minutes, or arc seconds does the Moon move in 1 hour of time?

The Moon lies roughly 384,000 km from Earth, and the Sun lies 150,000,000 km away. If both have the same angular size as seen from Earth, roughly how many times larger than the Moon is the Sun?

The stars in a constellation are physically close to one another.

The seasons are caused by the precession of Earth's axis.

If the direction of Earth's rotation were reversed, the solar day would be shorter than it is now.

The Moon does not rotate.

A lunar eclipse can occur only during a full Moon.

The angular diameter of an object is inversely proportional to its distance from the observer.

Once proved, a theory can never be changed.

The [blank_start]universe[blank_end] is the totality of all space, time, matter, and energy.

[blank_start]Astronomy[blank_end] is the study of the universe.

Planet [blank_start]Earth[blank_end] orbits a star called the [blank_start]Sun[blank_end], near the edge of the [blank_start]Milky[blank_end] [blank_start]Way[blank_end] Galaxy, which is just one among billions of galaxies in the [blank_start]universe[blank_end].

Early observers grouped the thousands of stars visible to the naked eye into patterns called [blank_start]constellations[blank_end], which they imagined where attached to a vast celestial sphere centered on Earth.

A [blank_start]celestial[blank_end] [blank_start]sphere[blank_end] is an imaginary sphere surrounding Earth to which all objects in the sky were once considered to be attached.

[blank_start]Constellations[blank_end] have no physical significance, but are still used to label regions of the sky.

The points where Earth's axis of rotation intersects the celestial sphere are called the north and south [blank_start]celestial[blank_end] [blank_start]poles[blank_end].

The line where Earth's equatorial plane cuts the celestial sphere is the [blank_start]celestial[blank_end] [blank_start]equator[blank_end].

The nightly motion of the stars across the sky is the result of Earth's [blank_start]rotation[blank_end] on it axis.

A [blank_start]rotation[blank_end] is the spinning motion of a body about on its axis.

The time from one noon to the next is called a [blank_start]solar[blank_end] day.

The time between successive risings of any given star is one [blank_start]sidereal[blank_end] day.

Because of Earth's [blank_start]revolution[blank_end] around the Sun, we see different stars at night at different times of the year, and the Sun appears to move relative to the stars.

The Sun's apparent yearly path around the celestial sphere (or the plane of Earth's orbit around the Sun) is called the [blank_start]ecliptic[blank_end].

We experience [blank_start]seasons[blank_end] because Earth's rotation axis is inclined to the ecliptic plane.

At the [blank_start]summer[blank_end] [blank_start]solstice[blank_end], the Sun is highest in the sky and the length of the day is greatest.

At the vernal and autumnal [blank_start]equinoxes[blank_end], Earth's axis of rotation is perpendicular to the line joining Earth to the Sun, so day and night are of [blank_start]equal[blank_end] length.

The interval of time from one vernal [blank_start]equinox[blank_end] to the next is one [blank_start]tropical[blank_end] year.

[blank_start]Precession[blank_end] is the slow "wobble" of Earth's axis due to the influence of the [blank_start]Moon[blank_end].

As a result of [blank_start]precession[blank_end], the [blank_start]sidereal[blank_end] year is slightly longer than the [blank_start]tropical[blank_end] year, and the constellations migrate around the calendar over the course of thousands of years.

The [blank_start]Moon[blank_end] emits no light of its own, but instead shines by reflected sunlight.

As the Moon orbits Earth, we see [blank_start]lunar[blank_end] phases as the amount of the Moon's sunlit face visible to us varies.

At [blank_start]new[blank_end] Moon, the Moon and the Sun are in nearly the same part of the sky, and the [blank_start]sunlit[blank_end] side of the Moon is oriented away from us.

At [blank_start]full[blank_end] Moon, the Sun and Moon are in opposite directions, from our perspective.

The time taken for the Moon to complete one orbit relative to the stars is 1 [blank_start]sidereal[blank_end] month. The time needed for the Moon to complete one cycle of phases is 1 [blank_start]synodic[blank_end] month.

A [blank_start]lunar[blank_end] eclipse occurs when the Moon enters Earth's shadow.

A [blank_start]solar[blank_end] eclipse occurs when the Moon passes between Earth and the Sun.

An eclipse may be [blank_start]total[blank_end] if the body in question (Moon or Sun) is completely obscured, or [blank_start]partial[blank_end] if only a portion of the surface is affected.

If the Moon happens to be too far from Earth for its disk to completely hide the Sun, an [blank_start]annular[blank_end] eclipse occurs.

Because the Moon's orbit around Earth is slightly inclined with respect to the [blank_start]ecliptic[blank_end], solar and lunar eclipses do not occur every [blank_start]month[blank_end].

The [blank_start]scientific[blank_end] [blank_start]method[blank_end] is a methodical approach employed by scientists to explore the universe around us in an objective manner.

A [blank_start]theory[blank_end] is a framework of ideas and assumptions used to explain some set of [blank_start]observations[blank_end] and construct theoretical models that make [blank_start]predictions[blank_end] about the real world. These predictions in turn are [blank_start]amenable[blank_end] to further observational [blank_start]testing[blank_end]. In this way, the theory expands and [blank_start]science[blank_end] advances.

Observation, theory, and testing are the cornerstones of the [blank_start]scientific[blank_end] [blank_start]method[blank_end].