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?

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3.1 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.

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Latitude and Longitude Used to locate positions on Earth.

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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.

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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).

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Illumination in June January 17, 2006 Astronomy 2010 6

Illumination in December January 17, 2006 Astronomy 2010 7

3.3 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.

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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.

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3.3.2/3 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.

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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.

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3.4 The Calendar • The challenge is that the obvious measures of day, month, and year don’t mesh.

– 1 lunar month = 29.5306 days – 1 solar year = 365.2422 days • Many early calendars, as evidenced by ruins found around the globe.

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Stonehenge January 17, 2006 Astronomy 2010 13

Mayan Observatory at Caracol January 17, 2006 Astronomy 2010 14

The Gregorian Calendar • The Romans used the leap year, counting 1 extra day every fourth year – 1 year  365.25 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!

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3.5 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, 2006 Astronomy 2010 16

Phases of the Moon January 17, 2006 Astronomy 2010 17

3.5.2 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”.

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The Moon Rotates Once per Revolution January 17, 2006 Astronomy 2010 19

3.6 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?

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The Moon’s Gravity Pulls Differently at Different Locations January 17, 2006 Astronomy 2010 21

Tides Come from Water Moving in Response to the Moon January 17, 2006 Astronomy 2010 22

The Sun’s Gravity Also Contributes: Alignment of Sun and Moon January 17, 2006 Astronomy 2010 23

3.7 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.

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

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The Moon’s Shadow Falls on the Earth January 17, 2006 Astronomy 2010 26

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

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