Chapter 2: Discovering the Universe for Yourself Flashcards

Flashcards testing core concepts from Chapter 2: Patterns in the Night Sky, Seasons, the Moon, and Planetary Motion.

How many stars are visible to the naked eye under good viewing conditions, and how many official constellations cover the sky?

You can see more than $2000$ stars with the naked eye, and there are $88$ official constellations that fill the entire sky.

Do the brightest stars in a constellation all lie at the same distance from Earth?

No, the brightest stars in a constellation may actually be quite far away from each other at varying distances from Earth.

What is the ecliptic on the celestial sphere?

The ecliptic is the Sun's apparent annual path around the celestial sphere.

How does our view change when looking into the galactic plane compared to looking out of it?

When looking into the galactic plane, we see the stars and interstellar clouds that form the Milky Way band. When looking out of the galactic plane, we have a clear view to the distant universe.

What three main reference points define an observer's local sky?

1. Zenith: The point directly overhead at an altitude of $90^{\circ}$.
2. Horizon: All points at an altitude of $0^{\circ}$ along the boundary of the visible sky.
3. Meridian: The imaginary line passing through the zenith and connecting due north and due south points on the horizon.

How is a full circle divided into smaller angular units of arcminutes and arcseconds?

A full circle equals $360^{\circ}$. Each degree contains $60'$ (arcminutes), and each arcminute contains $60''$ (arcseconds).

If your finger has an angular size of about $1^{\circ}$ at arm's length, how many arcseconds is that?

$3600''$ (arcseconds).

Why do stars appear to rise in the east and set in the west?

Because Earth rotates on its axis from west to east, making stars appear to circle from east to west.

What are circumpolar stars?

Stars near the north celestial pole whose daily circular paths remain entirely above the local horizon, meaning they never set.

What celestial point is located at the center of the circular star trails shown in this image?

The north celestial pole (which lies within $1^{\circ}$ of Polaris).

How are terrestrial coordinates latitude and longitude defined?

Latitude measures position north or south of Earth's equator ($0^{\circ}$). Longitude measures position east or west of the prime meridian ($0^{\circ}$), which passes through Greenwich, England.

How does the altitude of the celestial pole in your local sky relate to your location on Earth?

The altitude of the celestial pole in your local sky is equal to your latitude.

What star pattern in the Southern Hemisphere points toward the south celestial pole?

The Southern Cross points to the south celestial pole (which is not marked by any bright star).

What is the difference in length between a solar day and a sidereal day?

A solar day is $24$ hours long, whereas a sidereal day (Earth's true rotation period relative to distant stars) is $23$ hours and $56$ minutes long.

What is the primary cause of Earth's seasons?

The tilt of Earth's rotation axis causes sunlight to strike different hemispheres more directly during summer and less directly during winter as Earth orbits the Sun.

Why is Earth's varying distance from the Sun not the reason for seasons?

Earth's distance from the Sun varies by only about $3\%$, which is too small to cause large temperature changes, and seasons are opposite in the Northern and Southern Hemispheres.

How do the Sun's daily paths across the sky differ during solstices and equinoxes in the Northern Hemisphere?

On the summer solstice, the Sun follows its highest path and rises/sets furthest north of due east/west. On the winter solstice, it follows its lowest path and rises/sets furthest south. On equinoxes, the Sun rises precisely due east and sets precisely due west.

What is axial precession and how long is Earth's precession cycle?

Precession is the slow wobble of Earth's axis of rotation (similar to a spinning top). One full precession cycle takes about $26,000$ years.

How long does the Moon take to complete one orbit around Earth versus one complete cycle of phases?

The Moon's orbit around Earth takes $27.3$ days, while the complete cycle of phases (synodic month) takes $29.5$ days.

What is the difference between waxing and waning lunar phases?

Waxing phases get fuller, occur as the Moon moves from new to full, and are visible in the afternoon/evening. Waning phases get smaller, occur as the Moon moves from full to new, and are visible in late night/morning.

Why do we always see the same face of the Moon from Earth?

Because of synchronous rotation: the Moon rotates on its axis in exactly the same time it takes to orbit Earth.

What two conditions must be met for an eclipse to occur?

1. It must be a full moon (for a lunar eclipse) or a new moon (for a solar eclipse).
2. The Moon must be at or near one of the two nodes where its orbit crosses the ecliptic plane.

By how many degrees is the Moon's orbit tilted relative to the ecliptic plane?

The Moon's orbit is tilted by about $5^{\circ}$ to the ecliptic plane.

What is the saros cycle?

An $18$-year, $11\frac{1}{3}$-day cycle over which eclipse patterns recur.

Which five planets were known to ancient observers without telescopes?

Mercury, Venus, Mars, Jupiter, and Saturn.

What is apparent retrograde motion?

The temporary westward drift of a planet relative to the background stars over a few weeks or months, occurring when Earth passes (laps) an outer planet in its orbit.

What astronomical phenomenon is illustrated in this diagram?

Stellar parallax, which is the apparent shift in position of a nearby star against background distant stars as Earth orbits the Sun.

Why did most ancient Greeks reject the heliocentric (Sun-centered) model of planetary motion?

They could not detect stellar parallax with the naked eye and incorrectly believed that stars could not be far enough away to make the parallax invisible.