Sep 16 - Time-keeping and calendars

Question 1

Why study astronomy?

Answer 1

To measure time

Question 2

Measures of time based on astronomy (4)

Answer 2

§ 24 hours = time it takes for the sun to make one full circuit of the sky

§ Length of a month = moon’s cycle of phases

§ Year = based on cycle of seasons

§ Seven days = seven “planets” of ancient times: sun, moon, mercury, Venus, mars, Jupiter, Saturn

Question 3

Examples of ancient cultures’ methods of determining time of day:

Answer 3

• Observing the sun’s path through the sky, and shadows cast through using sticks or sundials
• Also estimate the time from the position and phase of the moon, or by observing the constellations visible at a particular time

Question 4

AM & PM

Answer 4

• AM = ante meridiem (before the middle of the day)
• PM = post meridiem (after the middle of the day)

Question 5

Examples of ancient cultures’ methods of determining seasons:

Answer 5

• Stonehenge
• Architectural alignments
• Sun daggers

Question 6

Solar Calendars:

Answer 6

one that is synchronized with the seasons so that seasonal events - such as the solstices and equinoxes - occur on approximately the same dates each year

Question 7

Lunar Calendars

Answer 7

aimed to stay synchronized with the lunar cycle, so that the moon’s phase was always the same on the first day of each month (through its 29.5 day cycle of phases)

• Lunar calendars have 12 months, with some lasting 29 days and others 30 ; the lengths are chosen to make the average agree with the approximately 2912-day lunar cycle.
• A 12-month lunar calendar therefore has 354 or 355 days, or about 11 days fewer than a calendar based on the Sun.
• The lunar phases repeat on the same solar dates about every 19 years (a pattern known as the Metonic cycle)

Question 8

Earth’s rotation period is actually 4 minutes short of 24 hours - why?

Answer 8

the daily circling of the stars in our sky is an illusion created by Earth’s rotation

You can therefore measure Earth’s rotation period by measuring how long it takes for any star to go from its highest point in the sky one day to its highest point the next day; AKA a sidereal day (23 hours, 56 minutes)

Question 9

What do we call a 24 hour day?

Answer 9

Solar Day: Based on the time it takes for the SUN to make a circuit around the local sky; from its highest point in the sky one day to its highest point the next day - 24 hours on average

Question 10

What do we call a month based on the moon?

Answer 10

Synodic month - gets its name from the idea that the sun and moon “meet” in the sky with every new moon; 29.5 day cycle of phases

Question 11

What do we call a month based on the sun?

Answer 11

Earth’s motion around the Sun means that the Moon must complete more than one full orbit of Earth from one new moon to the next

The Moon’s true orbital period, or a sidereal month, is about 2713 days - describes how long it takes for the moon to complete an orbit relative to the positions of distant stars

Question 12

What do we call a year based on the stars?

Answer 12

Sidereal year: The time it takes Earth to complete one orbit relative to the stars

Question 13

What is our calendar (solar) based off of? How does this differ from sidereal?

Answer 13

Tropical Year: the cycle of the seasons, which we measure as the time from the March equinox one year to the March equinox the next year

About 20 minutes shorter than the sidereal year

Question 14

Tropical & Sidereal year differences based on precession:

Answer 14

The difference between the sidereal year and the tropical year arises from Earth’s 26,000-year cycle of axis precession

Precession not only changes the orientation of the axis in space but also changes the locations in Earth’s orbit at which the seasons occur.

Question 15

Sidereal Period (Planetary Periods)

Answer 15

the time the planet takes to orbit the Sun; again, the name comes from the fact that it is measured relative to distant stars

Question 16

Synodic Period (Planetary Periods)

Answer 16

is the time from when a planet is lined up with the Sun in our sky once to the next similar alignment

Question 17

Conjunction - More Distant Planets (Planetary Periods)

Answer 17

when the planets line up with the sun

Cannot be seen because it’s hidden by the Sun’s glare, and rises/sets with the sun in our sky

Question 18

Opposition - More Distant Planets (Planetary Periods)

Answer 18

appear exactly opposite the sun in our sky (behind earth)

Moves through the sky like the full moon, rising at sunset reaching the meridian at midnight, and setting at dawn

Question 19

Nearer Planets & Greatest Elongation (Planetary Periods)

Answer 19

Never has an opposition but instead has two conjunctions—an “inferior conjunction” between Earth and the Sun and a “superior conjunction” when the planet appears behind the Sun as seen from Earth

An inner planet also has two points of greatest (eastern/western) elongation, when it appears farthest from the Sun in our sky (basically beside the sun)

Transits: on rare occasions, we do indeed see Mercury or Venus appear to pass directly across the face of the Sun during inferior conjunction.

Question 20

3 methods of telling time of day:

Answer 20

1. Apparent Solar Time
2. Mean Solar Time
3. Standard, Daylight, and Universal Time

Question 21

Apparent Solar Time (3 methods of telling time of day)

Answer 21

Time based on the Sun’s actual position in the local sky, as is the case when we use a sundial

Before noon, when the Sun is climbing higher in the sky, the apparent solar time is ante meridiem

After noon, the apparent solar time is post meridiem (“after the middle of the day”), or P.M

Question 22

Mean Solar Time (3 methods of telling time of day)

Answer 22

Averaging the differences between the time your clock would read and the time a sundial would read (as the 24 hour solar day can vary in length throughout the year)

A clock set to mean solar time reads 12:00 each day at the time the Sun crosses the meridian on average. (The actual mean solar time at which the Sun crosses the meridian varies over the course of the year in a fairly complex way)

Result: on any given day, a clock set to mean solar time may read anywhere from about 17 minutes before noon to 15 minutes after noon (that is, from 11:43 A.M. to 12:15 P.M.) when a sundial indicates noon.

Mean solar time is actually more convenient than apparent solar time (the sundial time); once set, any clock can tell the time whenever - but sundials depend on weather

Question 23

Standard, Daylight, and Universal Time (3 methods of telling time of day)

Answer 23

standard time, which today divides the entire world into time zones

In general, the standard time in a particular time zone is the mean solar time in the center of the time zone, in which case local mean solar time within a 1-hour-wide time zone never differs by more than about a half-hour from standard time.

Between the second Sunday in March and the first Sunday in November (when sun is on the meridian), most of the United States changes to daylight saving time, which is 1 hour ahead of standard time

It is useful to have a single time for the entire Earth. This “world” time was chosen to be the mean solar time in Greenwich, England—the place that also defines longitude 0°. Today, this Greenwich mean time (GMT) is often called universal time (UT)

Question 24

Gregorian calendar - Leap Years

Answer 24

Because the length of a year is actually about 36514 days, to keep the seasons and the calendar synchronized, the Julian calendar introduced the leap year: Every fourth year has 366 days (the extra day is Feb. 29), rather than 365, so that the average length of the calendar year is 36514days.

1582: Gregorian calendar was much like the Julian calendar, with two important adjustments. First, Pope Gregory decreed that the day in 1582 following October 4 would be October 15. By eliminating the 10 dates from October 5 through October 14, 1582, he pushed the date of the March equinox in 1583 from March 11 to March 21

Today, the Gregorian calendar is used worldwide for international communication and commerce.