Transcript Naked-eye astronomy

Knowing the Heavens
Chapter Two
Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change
• Naked-eye astronomy
– the sort that requires no equipment but human
vision
• Extends far back in time
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British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids
Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)
Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth
The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.
Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month
Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.
It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth
Circumpolar
stars
• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth
•Who volunteers to be
the class rep?
dec
RA
•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called observer’s
zenith.
Two more types of angle and time
• Hour angle (HA) of an object is the angle
between the meridian on which the object is
situated and the (observer’s) celestial
meridian.
• Local Sidereal Time (LST) is the Right
Ascension of an observer’s celestial
meridian.
• LST = RA + HA
The Seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole
Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later
What seasons are NOT:
• Seansons are NOT! caused by the changing
distance of the Earth from the sun during
the course of a year!
Sept
21
June
21
Dec
21
March
21
• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice
Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year
The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation
Precession causes the gradual change of the
star that marks the North Celestial Pole
Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere
• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon
You watch Sirius set at about 2 AM in the
middle of February. About what time will it
set in the middle of March?
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A) At 10 pm
B) At midnight
C) At 2 am
D) At 4 am
Astronomical observations led to the
development of the modern calendar
• Day is based on Earth’s rotation
• Month is based on the lunar cycle
• Year is based on Earth’s orbit
Different types of “day”
• Apparent solar day: time between two upper
meridian transits of the sun.
• Mean solar day: time between two upper
meridian transits of the mean sun.
(~361deg rotation)
• Sidereal day:
time between two upper
meridian transits of the vernal equinox.
(360 deg rotation)
1 mean solar day : 24 h
1 sidereal day
: 23 h 56 m 4.091s
Different types of “year”
• Calendar year: integer number of mean solar days,
(365 or 366)
• Sidereal year: time for the sun to return to the
same position with respect to the stars (time
of one 360 deg orbit of the earth around the sun).
• Tropical year: time for the sun to return to the
vernal equinox.
1 sidereal year = 365.2564 mean solar days
1 tropical year = 365.2422 mean solar days
Calendars
• Caesar introduced the 365.25 days
calendar and thus the Leap Year (an
extra day, February 29, every year
divisible by 4) .
• However, this is 11m 14s longer than the
tropical year. This accumulates to 3
days in 4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule
was invoked (Gregorian calendar).
Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• (Local) Sidereal Time is the Right Ascension that is
located on an observer’s local celestial meridian.
• LST = RA + HA
Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.
The Earth’s orbit
• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.
The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.
The different types of ‘year’.
• Sidereal year (year with respect to the stars)
= 365d 6h 9m 10s
= 365.2564d mean solar days
• Tropical year (successive passages of the Sun
through the Vernal Equinox) is 365d 5h 48m 46s
(365.2422d) in length.
• Due to precession, the tropical year is 20m 24s
shorter than the sidereal year.
The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.