Astronomy 161 An Introduction to Solar System Astronomy Prof. Scott Gaudi

Lecture 6: Daily & Annual Motions

Key Ideas:

• Reflection of the Earth's Daily Rotation
• Circumpolar Stars

• Reflection of the Earth's Orbital Motion
• Ecliptic: The Path of the Sun
• Zodiacal Constellations
• Solstices & Equinoxes

Daily Motions

• Earth rotates once a day (24 hours)
• The sense of rotation is Eastward
• Facing North, rotation is towards the Right.

Apparent Paths

Their orientation depends on your latitude:

• At Equator: perpendicular to the horizon
• At Poles: parallel to the horizon
• Mid-Latitudes: Tilted by (90º–Latitude)

In Columbus: we are at 40º N, so the paths are tilted by 50 degrees from the horizon.

Circumpolar Stars

These are the Circumpolar Stars

• Ursa Major, Ursa Minor, & Draco are circumpolar constellations as seen from Columbus

The opposite pole's circumpolar stars never rise above your horizon.

• Ursa Major never rises for latitudes south of about 40° S

Summary of Daily Motions:

• Celestial objects to appear to rise in the East and set in the West.
• Apparent daily paths are parallel to the Celestial Equator.
• The inclinations of these paths relative to the horizon depends on the observer's latitude.
• Circumpolar objects are those always above or below the local horizon.

Annual Motion of the Sun

This apparent motion is a reflection of the Earth's annual orbit around the Sun.

The Ecliptic

• Great Circle projected onto the Celestial Sphere
• Tilted by approximately 23.5° from the Celestial Equator

This tilt is called the Obliquity of the Ecliptic

The Obliquity of the Ecliptic varies between 21.2 and 24.5° with a roughly 41000 year period:

• Now it is 23° 26' 16.85"
• In 2000 it was 23° 26' 21.42"

The Zodiac

• Many date from Babylonian times.
• These are familiar from astrological lore: Aries, Pisces, Aquarius, Capricorn, Sagittarius, Scorpius, Libra, Virgo, Leo, Cancer, Gemini, and Taurus.

The Zodical Constellations can be used as a kind of an astronomical calendar:

• See which Zodiacal constellation is the highest (on your celestial meridian) at midnight.
• The Sun is in the opposite constellation.

Solstices

Solstices occur twice a year in June and December:

• Summer Solstice: Maximum northern declination of the Sun (June).
• Winter Solstice: Maximum southern declination of the Sun (December).

Equinoxes

Equinoxes occurs twice a year, during March and September.

• Vernal Equinox: Sun crosses the Celestial Equator moving North (occurs in March)
• Autumnal Equinox: Sun crosses the Celestial Equator moving South (occurs in September)

Length of the Day

Vernal & Autumnal Equinoxes:

• Occur in March and September, respectively.
• The Sun rises due East and sets due West.
• Day and Night have equal lengths (12 hours each).

Summer Solstice:

• Occurs in June
• The Sun rises in the Northeast, and sets in the Northwest
• Day is longer than Night in the Northern Hemisphere
• Day is shorter than Night in the Southern Hemisphere

Winter Solstice:

• Occurs in December
• The Sun rises in the Southeast, and sets in the Southwest
• Day is shorter than Night in the Northern Hemisphere
• Day is longer than Night in the Southern Hemisphere

Summary of Annual Motions:

• The Ecliptic: Sun's path relative to the stars.
• The Obliquity of the Ecliptic: 23.5°
• Constellations of the Zodiac along the Ecliptic.
• Equinoxes: Sun crosses the Celestial Equator.
• Solstices: Sun at maximum North & South declination.
• Length of the day depends on the location of the Sun along the Ecliptic.