Lecture 6: Daily & Annual Motions

Key Ideas:

Daily Motions Annual Motions

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.

Apparent Paths

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

Their orientation depends on your latitude:


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In Columbus: we are at 40 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 local horizon.

These are the Circumpolar Stars


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


Summary of Daily Motions:

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

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.
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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:


The Zodiac

As the Sun moves along the Ecliptic as seen from Earth, it passes through 12 ancient constellations known as the Zodiac.

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


(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:

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.

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:

Summer Solstice:

Winter Solstice:


Summary of Annual Motions:

Annual Motions reflect the Earth's orbit around the Sun:
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Updated: 2007 September 17
Copyright Richard W. Pogge, All Rights Reserved.