Apparent/Actual Motions Summary Daily motion of sun (and nightly motion of stars) is due to Earth’s rotation. Local noon occurs for an observer on the Earth when the Sun reaches its highest point in the sky during that day That occurs when the Sun crosses the observer’s meridian, the line that runs due North to due South for the observer. Clock time 12pm matches local noon only for one assigned reference meridian within a time-zone. The moon orbits the Earth; Earth and the planets orbit the sun. Some constellations are not visible year-round As the Earth orbits the sun we can only see those constellations in a direction away from the Sun.

Apparent/Actual Motions Summary The Sun, Moon, and planets appear to move through different constellations throughout the year. From year-to-year, the Sun, Moon and planets appear to move through the same group of constellations: the Zodiac The line on the sky they follow is called the ecliptic. They follow the same line because the planets, including Earth orbit the Sun in about the same plane. We call that plane the ecliptic plane.

Daylight Savings Time Daylight savings time Determined by society Days are longer than nights in summer Spring forward: Now: extra daylight after work in summer Then (World War I): save energy See Essay 3 in text for more on DST, time-zones, calendars, leap years.

Apparent Retrograde Motion of the Planets: Mars Example APOD June 13, 2010, Tunc Tezel Dr. Stephen J. Daunt's Astronomy 161 web site at The University of Tennesee, Knoxville. more animations

Lines in the Sky In order to use the sky to measure time you need to measure the location of objects in the sky. We will look at two methods of measuring locations in the sky. Both methods require measuring angles. These methods have long been used not only for timekeeping but for navigation as well. But first we need to define some terms

The Earth and Sky From where we stand Earth appears flat and the sky appears like a dome above us. The point where the dome of the sky meets the Earth is called the horizon.

The Earth and Sky If an object is above the horizon it is visible. If it is below the horizon it is blocked from view by the Earth Stars, the Sun, Moon and planets all appear to rise above the horizon in the East and set below it in the West.

Earth’s Spin Axis The imaginary line connecting Earth’s North and South Poles is called the Earth’s rotation axis. The line around which the Earth spins. Extending this line into the sky points to the North and South Celestial Poles. The Celestial Equator is the extension of Earth’s Equator.

Earth’s Spin Axis and Polaris Everything in the sky appears to move around the Celestial poles. The star Polaris lies very near the North Celestial Pole. It never rises or sets. It is always visible from the Northern Hemisphere. Polaris, the North Star, is never visible from the Southern Hemisphere. Polaris, the North Star A Hemisphere is half a sphere. The Northern Hemisphere is the half of the sphere north of the equator. The Southern Hemisphere is the half of the sphere south of the equator. Polaris is also known as the North Star. It is located in the tail of the Little Dipper. The Little Dipper is usually very hard to pick out in light-polluted skies. Polaris itself is not particularly bright either. Polaris will not always be the North Star. The Earth’s axis has a very slow wobble which causes it to point to different stars over time. This wobble however is so slow that it takes 26,000 years for it to wobble around one time. In 12,000 years however the North Star will be the star Vega and not Polaris. This slow wobble is known as precession.

It is the spin of the Earth on its axis that makes the Sun and stars appear to move across the sky.

Motion of Stars at the North Pole

Motion of Stars at the Equator

How to find the North Star Polaris can be found using the “Pointer” stars of the Big Dipper The Big Dipper is part of a larger constellation of stars called Ursa Major (the Great Bear) Now you know how to find true North. Pointer stars

Meridian, Altitude & Zenith Another important point in the sky is the Zenith, the point directly overhead. The line that crosses from North to South Horizon and passes through the Zenith and Poles is the Local Meridian. The Sun, Moon, star or planet appears at its highest point in the sky when it is on your local meridian. Local noon is the time when the Sun is at your local meridian. The morning hours are times before the Sun crosses the meridian. These are the hours of ante meridiem (before meridian) or AM The afternoon hours are times after the Sun has crossed the meridian. These are the hours of post meridiem (after meridian) or PM.

Altitude and Azimuth We can measure the location of objects in the sky using pairs of angles. Here’s one way. Altitude is the angle above the horizon. Azimuth is the angle around the horizon clockwise from North. North is 12:00, East is 3:00 If you picture the face of a clock then we can describe angles using different times. The angle is the separation between the hour hand and the minute hand. At 3:00 the two hands are separated by an angle of 90 degrees. At 6:00 the hands are separated by an angle of 180 degrees. At 9:00 you can measure the angle as being 270 degrees in the clockwise direction or 90 degrees if you measure counterclockwise. Finally at 12:00 the two hands are together, there are 0 degrees separating them. A full sweep around the clock face back to the beginning would be equal to 360 degrees.

Locations on Earth We identify positions on Earth using Latitude (Degrees North or South of the Equator) and Longitude (Degrees East or West of Greenwich, England). Greenwich is at the Prime Meridian (Longitude = 0°) PGCC about (39° North, 77° West)

Navigation Measuring the altitude of the North Celestial Pole can give you your Latitude on Earth in the Northern Hemisphere. South of the Equator Polaris is never visible. The farther North you are the higher the North Celestial Pole appears in the sky. Where would Polaris appear if you were standing at the North Pole? How would stars appear to move over the course of one night?

Locations in the Sky We can measure locations in the sky in a similar way. Like Latitude, the Declination is the angular distance from the Celestial Equator. Now we have to decide on a Celestial “Prime Meridian”. Astronomers choose one of the points where the Ecliptic crosses the Equator. The Sun moves along the Ecliptic. The Ecliptic is tilted to the Celestial Equator by 23.5 degrees.

Right Ascension & Declination The Sun traveling along the Ecliptic crosses the Equator at two points, the Vernal Equinox and the Autumnal Equinox. The Right Ascension is the angular distance from the Vernal Equinox Each position can be identified by its Right Ascension and Declination.