chapter 4.2; the seasons & 4.3 keeping time & 4.4 the calendar

seasons

divisions of the year, each with its different amount of sunlight

• difference between seasons gets more pronounced the father north or south from the equator

• seasons in the Southern Hemisphere are the opposite from the Northern Hemisphere

• caused by the 23.5 degree tilt of Earth’s axis

Earth’s titled axis

axis is tilted by 23.5 degrees

• Earth’s axis continues to point in the same direction in the sky throughout the year

• As Earth travels around the Sun, in June the Northern Hemisphere “leans into” the Sun and is more directly illuminated. In December, the situation is reversed: the Southern Hemisphere leans into the Sun, and the Northern Hemisphere leans away

• Spring and Autumn, the Earth leans sideways - the two hemispheres are equally favoured by the Sun

How does the Sun’s favoring one hemisphere translate into making it warmer for us down on the surface of Earth?

• when we lean into the Sun, sun light hits us at a more direct angle and is more effective at heating Earth’s surface

• increased daylight hours in summer and decreased daylight hours in winter

  • In June, the Sun is north of the celestial equator and spends more time with those who live in the Northern Hemisphere and vice versa

summer solstice (Northern Hemisphere)

21st June

• the Sun shines down most directly upon the Northern Hemisphere of Earth

• Sun appears approx. 23 degree north of the equator, therefore passes through the zenith of places on Earth that are at

• Sun’s rays shine down all around the North Pole

  • all places within 23 degrees of the pole will have sunshine for 24 hours

  • all places within 23 degrees of the South Pole will receive no sunlight (south of the Antarctic Circle)

winter solstice (Northern Hemisphere)

December 21

• latitude 23 degrees S, Tropic of Capricorn, the Sun passes through the zenith at noon

• Arctic Circle has no sunlight

• Antarctic Circle has 24 hours of sunlight

seasons at different latitudes

• near the equator; all seasons ae much the same

  • every day of the year there are approx. 12 hours of sunshine and 12 hours of night

  • wet and dry seasons

• at the North Pole, all celestial objects that are north of the celestial equator are always above the horizon, as Earth turns, circle around parallel to it

  • the Sun is north of the celestial equator from about March 21 to September 21, so at the North Pole, the Sun rises when it reaches the vernal equinox and sets when it reaches the autumnal equinox

• each year there are 6 months of sunshine at each pole, followed by 6 months of darkness

refraction

the bending of light passing through air or water

• due to the Earth atmosphere

• allows us to see “over the horizon”

• Because of this atmospheric refraction (and the fact that the Sun is not a point of light but a disk), the Sun appears to rise earlier and to set later than it would if no atmosphere were present

morning twilight

begins when the Sun is 18 degrees below the horizon

• the scatter of light by the atmosphere that occurs even when the Sun is below the horizon

evening twilight

extends until the Sun sinks more than 18 degrees below the horizon

• the scatter of light by the atmosphere that occurs even when the Sun is below the horizon

solar day

rotation period of Earth with respect to the Sun

• approx. 4 minutes longer than a sidereal day

sidereal day

the rotation period of Earth with respect to the stars

• solar day is slightly longer than a sidereal day because Earth not only turns but also moves along its path around the Sun in a day

• because our ordinary clocks are set to solar time, stars rise 4 minutes earlier each day

why is a solar day longer than a sidereal day?

• Suppose we start when Earth’s orbital position is at day 1, with both the Sun and some distant star (located in the direction indicated by the long white arrow pointing left), directly in line with the zenith for the observer on Earth

• When Earth has completed one rotation with respect to the distant star and is at day 2, the long arrow again points to the same distant star

• However, notice that because of the movement of Earth along its orbit from day 1 to 2, the Sun has not yet reached a position above the observer

• To complete a solar day, Earth must rotate an additional amount, equal to 1/365 of a full turn

• The time required for this extra rotation is 1/365 of a day, or about 4 minutes. So the solar day is about 4 minutes longer than the sidereal day.

apparent solar time

time reckoned by the actual position of the Sun in the sky (or, during the night its position below the horizon)

• time indicated by sundials

• middle of the night as the starting point in each day

• during the first half of the day, the Sun has not reached the local meridian (the great circle in the sky that passes through our zenith); before midday (a.m. ante meridiem)

• after sun reaches the local meridian (p.m. post meridiem)

• exact length of an apparent solar day varies slightly during the year

  • The eastward progress of the Sun in its annual journey around the sky is not uniform because the speed of Earth varies slightly in its elliptical orbit

  • Another complication is that Earth’s axis of rotation is not perpendicular to the plane of its revolution

  • thus, apparent solar time does not advance at the same rate (not a great fundamental unit of time)

mean solar time

based on the average value of the solar day over the course of the year

• contains exactly 24 hours and is what we use in our everyday timekeeping

• advantage: progresses at a uniform rate

• disadvantage: inconvenient for practical use because it is determined by the position of the Sun (e.g. exact time of noon is different as you change longitude)

daylight saving time

local standard time of the place plus 1 hour

• prolongs sunlight into evening hours

International Date Line

solution to the following dilemma:

• if you advance eastward around the world, every 15 degrees longitude you travel (each time zone), you set your watch an hour ahead

• by the time you have completed your trip, you will have gained a day

• runs approx. along the 180 degrees meridian of longitude

  • runs down the middle of the Pacific Ocean

  • at the date line, the date of the calendar is changed by one day

traditional functions of a calendar

• must keep track of time over the course of long spans, allowing people to anticipate the cycle of the seasons and to honour special religious or personal anniversaries

• to be useful to a large number of people, a calendar must use natural time intervals that everyone can agree on

  • defined by motions of Earth, the Moon, and sometimes planets

natural units of our calendar

• day: based on the period of rotation of Earth

• month: based on the cycle of the Moon’s phases

• year: based on the period of the revolution of Earth about the Sun

Julian calendar

introduced by Julius Caesar

• year: 365.25 days (actual value is 365.2422)

Gregorian calendar (Pope Gregory XIII)

corrected the issue with the Julian calendar (the true year differs by approx. 11 minutes)

• 10 days dropped to bring vernal equinox back to March 21

• change in rule for the leap year; three of every four century years (leap years under Julian calendar) would become common years; only century years divisible by 400 would be leap years

• countries in Eastern Europe did not adopt this calendar until later (Russian Rev dates)