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Chapter 3Earth, 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 SkyDirections 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 LongitudeUsed 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

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Illumination in December

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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 DayDuring one day, the Earth movesaround the Sun by about 1/365since a complete orbit is 365 days.This causes the time for the Earthto rotate back to face the Sun to belonger by about 4 minutes compardto the time to rotate back to face thesame 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

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Mayan Observatory at Caracol

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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

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Phases of the Moon

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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

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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

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Tides Come from Water Moving in Response to the

Moon

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The Sun’s Gravity Also Contributes: Alignment of

Sun and Moon

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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 EclipseOn 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

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A Lunar Eclipse: the Earth’s Shadow Falls on

the MoonLunar eclipses only occur during a full moon.