Download Lecture 1: Our Place in Space

Lecture Series 1: Basic Facts
• What is the universe?
– Answer: All of space, time, matter, and
energy.
• What is Astronomy?
– The investigation of the the universe or
cosmos.
Lecture Series 1: Scale
• What do scientist mean by scale?
– Class provide Dr. Connolly with an answer.
• How fast does light travel?
– 300, 000 km/s
• What is a light year?
– The distance light travels within one standard Earth
year. How long is that, be exact?
• 365.256 mean solar days - 1 Ly
Lecture Series 1: Scale
• How far does light travel in a year?
– One light year = 10 trillion kilometers
– That is ~ 6 trillion miles, for those who need
to think in miles.
Lecture Series 1: Distance
• 1 Ly or light year = the distance light travels in
one standard Earth year, or 10 trillion km or 6
trillion miles.
• If the distance to the nearest star is 4 Ly, than 4
x 10 trillion km = 40 trillion km.
Lecture Series 1: Distance
• What is a parsec?
– An astronomical unit of distance that is = 3.262 Ly
– The term parsec is derived from “parallax in arc
seconds”
• This can be related by the equation
– Distance (in parsec) = 1/parallax (in arc seconds)
Lecture Series 1: Distance
• Another unit of measurement for distance in
the cosmos is based on triangulation. To find
the distance to a point in two-dimensional
space, one needs three points. This come from
the discipline of trigonometry.
• This process forms the bases of Parallax.
Lecture Series 1: Distance
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Lecture Series 1: Distance
• Parallax = apparent
displacement of a
foreground object
relative to the
background of an
observer.
• The closer the object
the larger the
parallax.
Lecture Series 1: Distance
• Astronomers also use angular measurement. This is not always an
easy system to think within and has nothing to do with time.
•
– One circle contains 360o of measurement. Within that are 180o, 90o,
45o etc. with 180o = to the horizon.
1o can is subdivided into fractions of degrees termed arc minutes. There
are 60 arc minutes (60’) and an arc minute is further divided into 60 arc
seconds (60”). The Sun and Moon have an angular size of 30 arc minutes
of the horizon.
– Thus 1/60 of a degree = 1 arc minute (60’)
– Thus 1/3600 of a degree - 1 arc second (60”)
– Arc seconds are really small! Angular size depends on the actual size of an
object and its distance.
Lecture Series 1: Distance
• Astronomers also use a system of celestial
coordinates to determined positions of objects
in the cosmos.
– Declination is similar to latitude, units = degrees
– Right ascension is similar to longitude, units = time
Lecture Series 1: Distance
• Within the Solar System or smaller areas of the
Universe, how is distance determined or
measured?
– The easiest unit is the Astronomical Unit or AU.
– 1 AU is defined as the distance from the Earth to the
Sun. That is ~ 150,000,000 km or ~ 93,000,000 miles.
– There are ~ 63,240 AU in 1 Ly or light year.
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Lecture Series 1: Constellations
• What are
constellations?
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– A creation of the
human mind.
– Interesting but not
important to our
voyage through the
cosmos.
Lecture Series 1: Earth’s Movement
• Celestial Sphere
– No the stars are not
attached to the Earth.
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– But the Earth does rotate
around an axis ( tilt =
23.45o), and around a star
(tilt = 0.01o).
• Celestial equator
Lecture Series 1: Earth’s Movement
• Daily Changes
– These are due to the fact that the Earth rotates.
Time is determined by the Earth’s rotation around
its axis.
• Solar Day = From Noon to Noon = 24 hr
• Sidereal Day = The time it takes for a star or
constellation to complete one cycle in the sky,
returning to the same place it was observed the
previous day. (sidus which is Latin = star) 23hr
56min.
Lecture Series 1: Time
• Solar vs. Sideral Day
• Earth revolves around
the Sun (30 km/s) in
365.256 days, but around
its axis (360o; 0.5 km/s) in
24 hr. In one day the
Earth rotates 360o AND
moves through space.
Lecture Series 1: Earth’s Movement & Time
• How long is a standard solar
year?
– This is known as a sidereal
year, which is
~365.256363051 solar days.
– It does change ever so
slightly from year to year.
• What is one tropical year?
– From one vernal equinox to
the next, ~365.242 solar days.
– This is what our calendars
are based upon - Gregorian.
– If not, seasons would change!
Lecture Series 1: Earth’s Movement & Seasons
•
Why does Earth experience
seasons?
– Simple, the Earth is tilted AND it
revolves around the Sun.
– Seasons result from the changing
height of the sun above the
horizon.
– Ecliptic = apparent motion of
the Sun in the sky- Earth’s
orbit around the Sun.
•
What are the important dates
and names for seasons?
Lecture Series 1: Earth’s Movement & Seasons
•
Ecliptic = apparent motion of the
Sun in the sky- Earth’s orbit
around the Sun.
•
= A geometric plane within our
solar system that contains the
orbit of the Earth and most other
planets around the Sun.
•
= An imaginary plane that
contains the Earth’s orbit around
the Sun.
•
= Through the course of a
standard Earth year the Sun’s
path as seen from Earth lines
within a plane.
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Lecture Series 1: Earth’s Movement & Seasons
Lecture Series 1: Earth’s Movement & Seasons
• The two points where the
ecliptic intersects the celestial
equator are known as
equinoxes.
– At these places night and day
on Earth are the same
duration. Such dates are
known as Spring (vernal)
and Autumn (autumnal) !
– In the Christian Church
Spring = Eostar and Autumn
= Mabon
Lecture Series 1: Earth’s Movement & Seasons
• The point on the ecliptic
where the Sun is at its
northernmost point above the
celestial equator is known as
the summer solstice (sol which
is Latin = sun and stare which
is Latin = to stand) .
– In the Northern hemisphere
it is the longest day of the
year.
– Known as Litha in Christian
Church.
Lecture Series 1: Earth’s Movement & Seasons
• The point on the ecliptic
where the Sun is at its
southernmost point below the
celestial equator is known as
the winter solstice.
– In the Northern hemisphere
it is the shortest day of the
year.
– Known as Yule in the
Christian Church.
A Few Historical Facts
• From Celtic Calendars the following names were used
and evolved with time.
– Imbolic or Oimeaig = the beginning of Spring which is ~
Groundhog Day (Church = Candlemas).
– Beltane = Spring
– Lughnasadh = Autumn (Church Lammas)
– Samhain = Winter and the new year (Church = Hollowmas)
Lecture Series 1: Earth’s Movement & Seasons
• The 12 constellations that the
Sun passes as it moves along
the ecliptic are known as the
zodiac. These were of
important to astrologers of
old.
• In other words, the reason the
stars and constellations
change with the season is
because the cosmos moves as
we do through it.
Lecture Series 1: Earth’s Movement & Facts
• When the Earth is closest to
the Sun, it is termed the
perihelion (~ 2 January).
• When the Earth is farthest
from the Sun, it is termed the
aphelion (~2 July).
Lecture Series 1: Earth’s Movement & Lunar
• The Moon has phases or
changes in appearance during
the night sky because of how
the light from the Sun reflects
off the Moon as it rotates
around the Earth.
• New Moon, Full Moon, etc.
Lecture Series 1: Earth’s Movement & Lunar
Lecture Series 1: Earth’s Movement & Lunar
•
Sidereal month = 27.32 Earth
days. One complete revolution
around Terra.
•
Synodic month = 29.53 Earth
days. One complete cycle of
phases ---full moon to full moon.
Lecture Series 1: Earth’s Movement & Eclipses
•
A Lunar eclipse occurs when the
Earth comes between the Moon
and our star. The alignment is
perfect or nearly thus. Star light
no longer fully illuminates the
surface of the Moon. These can
be partial or total.
•
Such eclipses, when the Moon
passes through the Earth’s
shadow, last ~ 100 mins. Such an
eclipse is visible from all
locations on the Earth’s night
side.
Lecture Series 1: Earth’s Movement & Eclipses
•
A solar eclipse occurs when our
satellite, the Moon, passes
directly in front of the Sun. They
are very short in time, no more
than 7 mins for total cover.
•
During a total eclipse of the Sun,
the Moon is PERFECTLY
aligned between Earth and the
Sun (known as Umbra).
•
During a partial eclipse, the
Moon is not PERFECTLY
aligned (known as Penumbra).
Lecture Series 1: Earth’s Movement & Eclipses
•
Because the Moon’s orbit around
Earth is not perfectly circular, it
moves far enough away from the
Earth that the blockage is not
always complete, as in a total
eclipse. Such an eclipse is known
as a Annular eclipse.
Additional Readings
• I’ll have to think about this issue!