Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 5: Daily and Annual Motions

Key Ideas

Daily Motions:: Reflection of the Earth's Daily Rotation; Circumpolar Stars
Annual Motions:: Reflection of the Earth's Orbital Motion; Ecliptic: The Path of the Sun
Zodiacal Constellations

Daily Motions

Each day objects in the sky appear to: Rise in the East; Set in the West
This apparent daily motion is a reflection of the Earth's rotation about its axis: The Earth rotates onces a day (24 hours); The sense of rotation is Eastward
The word East originates from Old English and other words for 'dawn' and 'sunrise,' e.g. the Greek arion (morning) and Latin aurora (dawn).

Apparent Paths

The apparent paths of objects are parallel to the Celestial Equator
Their highest point in the sky is when they cross the Meridian
Their orientation depends on your latitude:: At Equator: perpendicular to horizon; At Poles: parallel to the horizon; Mid-latitudes: Tilted by (90 degrees - latitude)
In Columbus (40 degrees north): Paths are tilted 50 degrees from the horizon

Circumpolar Stars

A star closer than your latitude to your visible celestial pole (north or south) will always be above your horizon: These are the Circumpolar Stars; Ursa Major, Ursa Minor, and Draco are circumpolar constellations from Columbus
The opposite pole's circumpolar stars never rise above your horizon:: Ursa Major never rises for latitudes below 40 degrees South.

Annual Motion of the Sun

Over the course of a year:: The Sun appears to drift slowly Eastward each day relative to the stars.
Daily eastward drift is less than 1 degree per day: 365 days per year, but only 360 degrees in a circle.
This apparent motion reflects the Earth's annual orbit around the Sun.

Sidereal and Synodic Day

Sidereal Day: Time it takes the Earth to rotate 360 degrees on its axis.: Sidereal: Measured relative to the stars; A Sidereal Day is 23 hours 56 minutes and 4 seconds
Synodic Day: Time from one local noon to the next: Synodic: Measured relative to the Sun; A Synodic Day is 24 hours
The Earth orbits the Sun in 366.2422 sidereal days or 365.2422 synodic days.

The Ecliptic

Apparent path of the Sun relative to the stars.: Great Circle projected onto the Celestial Sphere; Tilted by 23.5 degrees from the Celestial Equator
This tilt is the Obliquity of the Ecliptic and changes slowly with time

The Zodiac

As the Sun moves along the Ecliptic as seen from Earth, it passes through 12 ancient constellations known as the Zodiac.: Many date from Babylonian times.; Familiar from astrological lore.
Can be used as an astronomical calendar:: See which Zodiacal constellation is on the celestial meridian at midnight.; The Sun is in the opposite constellation.

Solstices

A Solstice occurs when the Sun is at its maximum northern or southern declination along the Ecliptic.: From Latin sol sistit for "sun stands"
Summer Solstice:: Maximum northern declination of the Sun; Occurs in June; Sun is directly overhead on the Tropic of Cancer
Winter Solstice:: Maximum southern declination of the Sun; Occurs in December; Sun is directly overhead on the Tropic of Capricorn

Equinoxes

Equinoxes occur when the Sun crosses the Celestial Equator: From Latin equi noctis for "equal night"
Happens twice a year in March and September
Vernal Equinox:: Sun crosses the Celestial Equator Northward in March
Autumnal Equinox:: Sun crosses the Celestial Equator Southward in September

Length of the Day

Vernal and Autumnal Equinoxes:: Sun rises due East and sets due West; Day and Night are equal (12 hours)
Summer Solstice:: Sun rises in Northeast, sets in Northwest; Day is longer than Night.
Winter Solstice:: Sun rises in Southeast, sets in Southwest; Day is shorter than Night

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.