Keeping Track of Time

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 (Sun on Celestial Equator at beginning of Spring); approximately 20.4 minutes shorter than Sidereal Year. 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 poles Celestial Equator Position of Vernal Equinox These, in turn, mean that the celestial coordinates (celestial longitude and latitude) of an object change with time because the coordinate system moves

“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 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) 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 are lunar/solar hybrids. For example, the Jewish calendar periodically adds months to the year in order to keep pace with the seasons. The Islamic calendar is lunar based. It has 12 lunations (lunar months) per year. Since this is less than a tropical year, seasons do not fall at the same time each year but rather repeat on an ~33 year cycle.

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 the visible objects in the sky that move differently than the stars: Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn Object Roman Anglo-Saxon English Sun Solis Sun Sunday Moon Lunae Moon Monday Mars Martis Tiw Tuesday Mercury Mercurii Woden Wednesday Jupiter Jovis Thor Thursday Venus Veneris Freya Friday Saturn Saturni Saturn Saturday

Eclipses: understanding shadows

Lunar phases arise from viewing the illuminated half of the Moon from different orientations. The Earth & Moon are also casting shadows into space. What are the consequences of these shadows?

Lunar Eclipses If a light source is extended (like a large light bulb), any object casts a shadow that consists of a zone of partial shadow, the Penumbra, and a zone of full shadow, the Umbra. 15 15

Lunar Eclipses If the moon passes through Earth’s full shadow (Umbra), we see a lunar eclipse. If the entire surface of the moon enters the Umbra, the lunar eclipse is total. 16 16

Lunar Eclipse Three types: (1) Penumbral (2) Partial (3) Total

A Total Lunar Eclipse Motion of the moon against the background of stars Motion of the moon against the horizon 19 19

A total lunar eclipse can last up to 1 hour and 40 min. During a total eclipse, the moon has a faint, red glow, reflecting sun light scattered in Earth’s atmosphere. 20 20

This was early evidence that the Earth is spherical. Note that the edge of the Earth's shadow, as seen on the Moon during a partial eclipse, is curved. This was early evidence that the Earth is spherical.

At what lunar phase(s) can lunar eclipses occur?

Solar Eclipses The sun appears approx. as large in the sky (same angular diameter ≈ 0.50) as the moon. ≈ 0.5o = 30’ 23 23

Solar Eclipses‏  When the moon passes in front of the sun, the moon can cover the sun completely, causing a total solar eclipse. 24 24

The widest the Moon's umbral shadow gets at the Earth is ~270 km, while the penumbral shadow ~7000 km For this reason, total and annular eclipses of the sun last only a few minutes at most and are seen over only a very small area

Total Solar Eclipse Chromosphere and Corona Prominences 26 26

Annular Solar Eclipses The angular sizes of the moon and the sun vary, depending on their distance from Earth. When Earth is near closest to the sun, and the moon is nearest the Earth, we see an annular solar eclipse. Perigee Apogee Perihelion Aphelion 27 27

Line of Nodes - line connecting the two points where the Moon's orbit passes through the Ecliptic plane To have an eclipse, the Line of Nodes of the Moon's orbit must be lined up with Earth and Sun - “Eclipse Season”.

 Saros cycle: 18 years, 11 days, 8 hours Conditions for Eclipses Eclipses occur in a cyclic pattern.  Saros cycle: 18 years, 11 days, 8 hours 29 29

Conditions for Eclipses The moon’s orbit is inclined against the ecliptic by ~ 50. A solar eclipse can only occur if the moon passes a node near new moon. A lunar eclipse can only occur if the moon passes a node near full moon. 30 30

If the Moon's orbit maintained the same orientation in space, eclipse season would occur every half year. However, the Moon's orbit precesses. This leads to eclipse seasons separated by approximately 173 days.

Given the 29 ½ day lunar phase cycle (synodic period) and the ~ 173 day period of the eclipse season, there is an approximate concurrence of these cycles every ~18 yr 11 1/3 days This is called the Saros Cycle

Total solar eclipses

If you were on the Moon during a solar eclipse, what would you see? (a) A faint ruddy reddish Earth (b) Nothing different (c) A small circular shadow moving across the Earth's surface (d) A bright ring surrounding a darkened Earth.

If you were on the Moon during a solar eclipse, what would you see? (a) A faint ruddy reddish Earth (b) Nothing different (c) A small circular shadow moving across the Earth's surface (d) A bright ring surrounding a darkened Earth.