Homework #3 will be posted soon.

Several “out-of-class” activities will also be posted shortly.

Email announcements will be sent

TIME

24 HOUR DIVISION OF THE DAY

Around 1500 B.C., Egyptians developed a sundial, onto which they divided the daylight hours into 10 equal parts.

24 HOUR DIVISION OF THE DAY

Around 1500 B.C., Egyptians developed a sundial, onto which they divided the daylight hours into 10 equal parts.

They designated two additional parts (“hours”) to signify twilight time (morning & evening)

They divided the night time into twelve portions based upon crossing of the meridian by evenly spaced “clock stars”

Ever since then, we have divided the day into twenty-four portions (hours)

Why do clocks run in the “clockwise” direction?

Apparent Solar Time

vs

Mean Solar Time

Apparent Solar Time: Based on the location of the sun in the sky relative to the local meridian

Because of the Earth’s variable orbital speed (due to its noncircular orbit) and to the inclination of Ecliptic to Equatorial plane, the rate at which the sun appears to move is not uniform.

This leads to variable length days!

Not very useful to have hours and days that are not uniform in length!

Solution: Create fictitious sun which moves at a uniform rate equal to the mean motion of the sun.

Mean Solar Time

Location of “average” sun relative to the local meridian. This average sun moves at a constant speed relative to the

celestial equator – equal length days.

Mean solar time can run from 17 minutes earlier than apparent solar time to 15 minutes later.

Relationship between two is given by the “analemma”

Both apparent and mean solar time are defined locally. Need more uniform time keeping scheme.

Standard Time: Time zones within which the time is approximately the same as the mean solar time at the center of the zone.

The Year

Calendar

Important in societies that need to keep track of “annual” events, such as seasons

Based upon orbit of Earth about the Sun

Complicated by uneven number of days in year

Sidereal Year: Length of time required for Earth, Sun, and stars to return to same configuration.

Tropical Year: Length of time between successive Vernal Equinoxes.

These differ in length because of the precession of the Earth’s axis.

Precession The Earth’s axis “wobbles”, similar to a top, causing the direction of rotation to change with time.

The orientation returns to its original direction every 26,000 years.

Precession causes movement of:

Celestial polesCelestial EquatorPosition of Vernal Equinox

These, in turn, mean that the celestial coordinates (RA and declination) of an object change with time because the coordinate system moves

Must specify “epoch” of coordinates(e.g., 2000.0)

“Sun Signs” shift in sky – popular “signs” really relate to positions 2000 years ago

From historical and practical

perspectives, it is desirable to have

a calendar in which seasons fall at the same time

each year.

Egyptian Calendar: (~4200 BCE) – calendar of 365 days

but tropical year ~ 365 ¼ days in length, so seasons got out of sync.

Julian Calendar: (46 BCE). Introduced concept of leap year. One day added to calendar every four years. Spring set to March 24.

However, tropical year actually ~ 11 minutes short of 365 ¼ days. By late 1500s, the beginning of spring (Vernal Equinox) was falling on March 11.

Gregorian Calendar: (1582). Designed to maintain March 21 date of Vernal Equinox. Added leap centuries to calendar. Leap year for “hundred's year” only if century divisible by 400.

Oct. 1, 2, 3, 4, 15, 16 ... 1582

The Gregorian Calendar was adopted at different times around world; 1752 in England and American colonies, 1912 in China, 1919 in Russia.

Other calendars are in active use. Some of these are lunar/solar hybrids, e.g., the Jewish calendar periodically adds months to the year in order to keep pace with the seasons.

The year is divided into 12 months, based upon lunar cycles

Months are divided into Weeks:

* The week is traditionally divided into 7 days named for Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn

Object Roman Anglo-Saxon English Sun Solis Sun SundayMoon Lunae Moon MondayMars Martis Tiw TuesdayMercury Mercurii Woden WednesdayJupiter Jovis Thor ThursdayVenus Veneris Freya FridaySaturn Saturni Saturn Saturday

GROUP ACTIVITY

You will be shown the relative positions of the planets for today’s date, as seen from high above (to the north) of the solar system.

Keep in mind that the planets • all orbit in approximately the same plane, • all orbit in the same direction (prograde), • inner planets have shorter orbital periods than planets on larger orbits,

Given this, think about and answer the following questions.

1. Which planets are visible at 9 pm? At 3 am?

2. Mercury and Venus appear in the sky only shortly after sunset, at which time they are called “evening stars”, OR shortly before sunrise (“morning stars”). What are these two planets currently?

3. The orbit planes of all of the planets are near a plane for which we have already discussed. What is the name of this plane? What defines it?

4. Do we expect to ever see either the inferior planets (Venus & Mercury) or the inferior planets (all the rest) close to the North Celestial Pole? At southern celestial latitudes? Explain.

Saturn

Jupiter

Outer solar systemInner solar system

Venus

Earth

Mars

Mercury

Planets in the Sky

The five “naked eye” planets are very easy to

see –> bright <-

Planets Known in Ancient Times Mercury

difficult to see; always close to Sun in sky Venus

very bright when visible — morning or evening “star”

Mars noticeably red

Jupiter very bright

Saturn moderately bright

Close grouping of these five planets in April 2002.

Note that these planets plus the recently set Sun all lie essentially along a line . . . .

Where in the sky should we look to see the planets?

The planets are always seen near the ecliptic.

This is a consequence of the planets orbiting in planes that are near each other.

Retrograde Motion

(1) Planets, including the Earth, orbit the

Sun

(2) Planets closer to the Sun have shorter orbital periods than planets farther from

the Sun

As we “pass” a planet, it appears to move backwards

(as seen from Earth)

Parallax AngleApparent shift of a star’s position due to the Earth’s orbiting of the Sun

The ancient Greeks rejected the notion that the Earth orbits the sun. Why?

● It ran contrary to their senses.

● If the Earth revolved about the Sun, then there should be a “great wind” as we moved through the air.

● Greeks knew that we should see stellar parallax if we orbited the Sun – but they could not detect it.

Parallax AngleApparent shift of a star’s position due to the Earth’s orbiting of the Sun

The nearest stars are much farther away than the Greeks thought.

So the parallax angles of the star are so small, that you need a telescope to observe them.

Possible reasons why stellar parallax was undetectable:

1. Stars are so far away that stellar parallax is too small for naked eye to notice

2. Earth does not orbit Sun; it is the center of the universe

Unfortunately, with notable exceptions like Aristarchus, the Greeks did not think the stars could be that far away, and therefore rejected the correct explanation (1)…

Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems.

We have now set the stage for discussing the historical development of astronomy

Close grouping of five planets in April 2002.

Note that these planets plus the recently set Sun all lie essentially along a line . . . .

This is a pattern that was well known to the “ancients”

Locations of planets in the sky

Mercury: always close to Sun in sky

Venus: always close to Sun in sky

Mars: no restrictions on distance from Sun in sky

Jupiter: no restrictions on distance from Sun in sky

Saturn: no restrictions on distance from Sun in sky

What causes these differences?

On short term (diurnal motion), planets appear to move with the stars, east to west, making a full circuit around the sky (meridian to meridian) in approximately one day

Most of the time, planets move slowly eastward each day relative to the stars: different planets at different rates

Motions of the planets

What causes these motions?

Planets are always close to the “ecliptic”, the apparent annual path of the sun through the sky.

Close grouping of five planets in April 2002.

This is a pattern that was well known to the “ancients”

Why are the planets restricted to these locations?

Some planets occasionally reverse their motion relative to the stars, moving slowly westward relative to the stars, for a few days

apparent retrograde motion

What causes this?

What causes this?

What causes the observed motions of the stars, sun, moon, and planets in the sky?

The Greeks developed a model for the Universe that lasted for nearly 15

centuries.

It did a reasonably good job explaining these motions.

Claudius Ptolemy (100-170 CE)

Developed a model of the universe designed to fit the observational data.