Elenco Night’n Day® - Manual (Page 9)

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SEASONS AND DATES
The Earth turns once on its axis in 24 hours with respect to the Sun. This is the day.
The movement is called rotation.
The Earth is also moving in its orbit around the Sun. This movement is called
revolution and one complete revolution around the Sun is the definition of the year.
The Earth takes 365.24 days to orbit the Sun once. The 0.24 days (approximately one
quarter of a day) is why we have leap years almost every four years. After four years
that quarter day adds up to one day. Calendars have whole days, not halves or
quarters, so three of four years have 365 days, and one, leap year has 366.
Because the extra bit is not exactly 0.25 days, leap years are not necessarily every
four years. If a year is divisible by four it is a leap year. If it is a century year (1800,
1900, 2000 etc.) it is a leap year only if it can be evenly divided by 400. So 2000 was a
leap year and the year 1900 was not.
The axis of the globe is tilted by 23.5º from being vertical. This is the same as the
actual planet. This angle is called the Obliquity of the Ecliptic.
The Earth’s tilt is always in the same direction. This results in the axis of the Earth
pointing to the same place in the northern and southern skies. In the northern skies,
the star Polaris is in line with the axis, which is why it appears to stay in the same
place during the night as the Earth turns.
The tilt of the Earth is also what causes seasons. When the northern hemisphere is
tipped toward the Sun, summer happens. The southern hemisphere would then be
tipped away from the Sun, so winter occurs there. The seasons flip when six months
later the northern half of the Earth is pointed away from the Sun, and the southern
half pointed toward the Sun.
GLOSSARY
DAY
The time it takes for the Earth to make one rotation on its axis with respect to
the Sun. The solar day = 24 hours. When the day is referenced against the
stars, the star day or sidereal day is 23 hours 56 minutes.
ECLIPTIC
An imaginary line that the Sun traces against the sky as the year passes. As the
Earth moves, or revolves around the Sun, our orbital motion makes the Sun
appear to move against the stars.
FROM SPACE VIEW
ROTATION AND TRANSLATION OF EARTH AROUND THE SUN
ECL
IPTI C
WINTER
SUMMER
AUTUMN
SPRING
SUMMER
WINTER
SOLSTICE SOLSTICE
EQUINOXE
1. EQUATOR
2. TROPICS
3. POLAR CIRCLES
4 TIME ZONES F.V. MARCH 2009
EQUINOXE
SPRING
AUTUMN
September
October
November
December
January
February
March
April
May
June
July
August
1
2
2
3
3
4 4 4
EQUATOR
The line that divides the
northern and southern
hemispheres. It is halfway
between the two poles and
its latitude = 0 º. The Earth
is approximately 8,000 miles
in diameter and the
equator (or circumference
of the Earth) is about
25,000 statute miles. The
Earth’s rotational speed at
the equator is slightly more
than 1,000 miles per hour.
EQUINOX
The two times of the year when the poles of the Earth are neither tipped
toward nor away from the Sun. Day and night have the same length for all
locations non the Earth at the equinoxes (equi = equal, nox = night). The
equinoxes occur on March 21
st – 22 nd and September 22 nd – 23 rd .
SOUTH
CELESTIAL
POLE
Rotation Axis
Axial tilt
or
Obliquity
Perpendicular
to orbit
CELESTIAL EQUATOR
ECLIPTIC
Orbit direction
NORTH
CELESTIAL
POLE
Earth Axis
Sun r
ay
s
Arctic Circle
Tropic of Cancer
Equator
Tropic of Capricorn
Antarctic Circle
66˚ 33’ North
66˚ 33’ South
23˚ 27’ North
23˚ 27’ South
0˚