Lecture 6: Daily & 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
Solstices & Equinoxes

Daily Motions

Objects in the sky appear to rise in the East and set in the West each day. This apparent daily motion is a reflection of the Earth's rotation about its axis.

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

Apparent Paths

The Apparent Paths of objects are parallel to the Celestial Equator.

Their orientation depends on your latitude:

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

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

Circumpolar Stars

Any 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, & Draco are circumpolar constellations as seen from Columbus

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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:

Daily Motions of celestial objects reflect the Earth's daily rotation about its axis:

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

Over the course of a year, the Sun appears to move a little towards the East each day as seen with respect to the background stars. This daily eastward drift is <1° per day (there are 365 days in a year, but only 360° in a circle).

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

The Ecliptic

The Ecliptic is the apparent path of the Sun relative to the stars.

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

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This tilt is called the Obliquity of the Ecliptic

The Obliquity changes slowly over time:

Now (2005 Jan 10): it is 23° 26' 26.62"
In 1900 it was 23° 27' 05.97"

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.
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.

(Click on the image to view it in color at full resolution [Size: 21Kb], inspired by a drawing by Nick Strobel)

A Note about the Zodiac:

The amount of time the Sun spends in the region of a particular Zodiacal constellation does not, in fact, correspond to the calendar dates associated with the twelve "Sun Signs" in the newspaper and magazine astrology columns. This is no accident. As we will see in Lecture 7, the celestial equator slowly moves ("precesses") around the sky once every 26,000 years. This means that the celestial location of the Sun during a given month today has moved substantially since the common astrological traditions were codified by Ptolemy during the 2nd century AD. In fact, according to the constellation boundaries established by the IAU in 1888, at the present time the Sun actually passes through 13 constellations over the course of a year! In late November the Sun crosses over the northern part of Scorpius for about 5 days, after which it passes into the constellation of Ophiuchus until mid-December. Ophiuchus is, of course, not one of the traditional 12 constellations of the Zodiac. The amount of time the Sun spends in a constellation is also quite variable, and nowhere near the uniform time intervals suggested by traditional astrology. While both astronomy and astrology share common roots in the distant past, they bear no relation to each other today. Astrology as now practiced has strayed far from the reality clearly visible in the night sky.

Solstices

Solstices occur when the Sun is at its maximum northern and southern declination. The word Solstice is derived from the Latin words "sol sistere" = "sun" and "stand still".

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).

These names are ambiguous, as while in June it is Summer in the Northern Hemisphere, it is Winter in the Southern Hemisphere (and similarly flipped during December). You may occasionally see these called the "June" and "December" Solstices, respectively, in an attempt to not have a "Northern" bias, but this alternative is not widely used.

Equinoxes

Equinoxes occur when the Sun crosses the Celestial Equator. Derives from the latin "equinoctis" = "equal night".

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)

Like with the Solstices, there is a similar North/South ambiguity in their related seasons. In this case, however, the use of the latinate names (Vernal instead of Spring), signifies that we are using a strict astronomical definition that is agreed upon worldwide.

Length of the Day

The length of the day depends on the location of the Sun along the Ecliptic.

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Vernal & Autumnal Equinoxes:

In 2006 these occur on March 20 and September 22, respectively.
The Sun rises due East and sets due West.
Day and Night have equal lengths (12 hours each).

Summer Solstice:

Occured on June 21 in 2006.
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 on December 22 in 2006.
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:

Annual Motions reflect the Earth's orbit around the Sun:

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.

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