January 17, 2006Astronomy Chapter 3 Earth, Moon, and Sky How do we locate objects in the sky? How are seasons and tides related to astronomy? What goes into making a modern calendar?

January 17, 2006Astronomy Earth and Sky Directions on Earth: North (N) and South (S) are the directions to the corresponding poles. East (E) is the direction in which the Earth rotates and West (W) is opposite. Locations defined by latitude and longitude. Meridian: an imaginary circle around the Earth passing through the poles.

January 17, 2006Astronomy Latitude and Longitude Used to locate positions on Earth.

January 17, 2006Astronomy Seasons Arise from the Earth’s Tilt The Earth’s axis of rotation is tilted 23 degrees to its plane of rotation. At different times of the year more solar radiation (light) strikes the northern or southern hemisphere.

January 17, 2006Astronomy The Sun’s Path Changes with the Seasons The Sun is above the Cel. equator in summer, and below in winter (in the Northern hemisphere).

January 17, 2006Astronomy Illumination in June

January 17, 2006Astronomy Illumination in December

January 17, 2006Astronomy Keeping Time This is just a summary of the history of time. Our fundamental measure of time is the length of a day. –Our normal day is called a solar day – the Earth rotates once w.r.t. the Sun, or noon-to-noon. –The Earth moves from one sunrise to the next by about 1°, so stars appear to shift. –A sidereal day is the time for the Earth to rotate once w.r.t. the stars, and is about 4 minutes shorter.

January 17, 2006Astronomy Sidereal Versus Solar Day During one day, the Earth moves around the Sun by about 1/365 since a complete orbit is 365 days. This causes the time for the Earth to rotate back to face the Sun to be longer by about 4 minutes compard to the time to rotate back to face the same star.

January 17, 2006Astronomy /3 Solar Time Apparent solar time is the time at your location based on the position of the Sun. –It varies from city-to-city (Detroit to Baltimore). –It varies day-to-day because the speed of the Earth varies. Mean solar time averages out the day-to-day changes. Standard time is the same everywhere in a time zone, and changes by 1 hr. or ½ hr. between zones. Daylight saving time is standard time + 1 hr. Shifts daylight to align with work hours.

January 17, 2006Astronomy The International Date Line Traveling westward, a new time zone appears every 15° of longitude, each 1 hr. earlier. You lose 24 hrs. (1 day) when returning home. The problem is solved by the use of the international date line. It passes thru the Pacific Ocean, by agreement.

January 17, 2006Astronomy The Calendar The challenge is that the obvious measures of day, month, and year don’t mesh. –1 lunar month = days –1 solar year = days Many early calendars, as evidenced by ruins found around the globe.

January 17, 2006Astronomy Stonehenge

January 17, 2006Astronomy Mayan Observatory at Caracol

January 17, 2006Astronomy The Gregorian Calendar The Romans used the leap year, counting 1 extra day every fourth year –1 year  days –11 minutes of error every year By 1582 the calendar was off by 10 days. Pope Gregory XIII introduced a new calendar, and skipped 10 days so that Oct. 4, 1582 was followed by Oct. 15, 1582!

January 17, 2006Astronomy Phases and Motions of the Moon The Moon is the second brightest object in the sky, yet it’s all reflected sunlight. The Moon goes through phases every month. –New Moon –First quarter –Full Moon –Third quarter –Back to new

January 17, 2006Astronomy Phases of the Moon

January 17, 2006Astronomy The Moon’s Revolution and Rotation Revolution – to go around in the orbit. Rotation – to spin around an axis. The Moon revolves once in about 29 days. The Moon rotates once in exactly the same time. We see only one side of the Moon. The side we don’t see is called the “dark side”.

January 17, 2006Astronomy The Moon Rotates Once per Revolution

January 17, 2006Astronomy Ocean Tides and the Moon Tides are due primarily to the Moon, with some influence from the Sun. All points on the Earth are not equally distant from the Moon. –The pull of the Moon’s gravity is different. –Forces result that push water and raise tides. Each day there are 2 high tides and 2 low tides. Why?

January 17, 2006Astronomy The Moon’s Gravity Pulls Differently at Different Locations

January 17, 2006Astronomy Tides Come from Water Moving in Response to the Moon

January 17, 2006Astronomy The Sun’s Gravity Also Contributes: Alignment of Sun and Moon

January 17, 2006Astronomy Eclipses of the Sun and Moon An eclipse occurs when one object passes between the Sun and another object, blocking the light from the Sun. –Solar eclipse: the Moon moves between the Earth and the Sun. –Lunar eclipse: the Earth moves between the Moon and the Sun.

January 17, 2006Astronomy What’s a Solar Eclipse On Earth, the apparent size of the Moon and Sun are almost identical. Total solar eclipses are dramatic.

January 17, 2006Astronomy The Moon’s Shadow Falls on the Earth

January 17, 2006Astronomy A Lunar Eclipse: the Earth’s Shadow Falls on the Moon Lunar eclipses only occur during a full moon.