Transcript Astronomy

Astronomy
Unit 1
The celestial sphere and the seasons
Coordinate system
There are 2 coordinate systems that can be used to
locate things in the sky.
1. Horizon system: it is based on your location and
the momentary position of the object with
respect to the horizon.
2. The Equatorial system: it is based on an
imaginary set of grid lines starting with the
equator and the meridian. These objects will be
in the same positions at the same time of year.
Horizon system: key terms
Horizon system
Used to locate a star/planet using altitude
(distance above horizon) and azimuth
(direction) from a specific location.
Zenith
the point directly above you in the sky
Nadir
The point directly below you
Horizon
where the ”land” or tree-line meets the sky
Altitude
from 0º on horizon to 90º at zenith
Azimuth
from 0º at N, 90º at E, 180º at S, 270º at W
Notice that the position of stars change over the course of a few
minutes or hours because Earth is spinning
(the yellow streaks are fairies and fireflies)
Horizon system
Problems...
• 1. Stars change position over
the course of the night because
of rotation of Earth—stars rise
and set like the sun
• 2. Stars occupy different
positions in the sky at different
points on the earth (N vs. S
hemispheres)
Equatorial (Celestial) Coordinate System
• Universal system for all locations
• Declination: similar to altitude or lattitude,
degrees north or south of the equator
• Right Ascension: similar to azimuth or longitude,
direction east or west of the meridian.
– Measured in hours, minutes, and seconds because the
earth is spinning over time (think time zones)
• Based on the celestial sphere, an imaginary
projection of a sphere surrounding us where all
celestial objects are placed.
Celestial Sphere
Imaginary sphere onto
which all stars are
projected (imaginary
sphere of what we
see)
North Celestial Pole
Point on celestial
sphere directly
above North Pole of
earth
You can read more about it in your textbook on pg. 20
Polaris
The North Star because
it is practically
located on the North
Celestial Pole
Diurnal Circles from mid-latitude
Circumpolar stars
group of stars that do
not set below horizon
(they go around
Polaris)
Latitude and Polaris
The angle of Polaris
above your horizon is
the same as your
latitude in degrees.
Note that the stars are moving counter clockwise across the
sky.
If you are standing at the North Pole, your
view of the celestial sphere looks like this…
Remember: the altitude of Polaris is the same as your
latitude. Where would Polaris be if you were on the equator?
If you are standing at the equator, your
view of the celestial sphere looks like this…
If you are standing in Atlanta, your view of
the celestial sphere looks like this…
We can also measure objects on the sphere
• The apparent distance between two points on the
dome of the sky is the angular distance, which is
the angle between the two objects, measured with
the vertex at the center of the sphere (earth).
• Any angle measured with the vertex at the center is
said to be measured in degrees of arc.
– Fractional values can be specified in arcminutes and
arcseconds.
– Note that 1 degree of arc is always the same size on the
sphere of the sky, no matter where it is measured on the
"surface" Angular distance or angular size can be
measured in any direction on any part of the sky.
Measuring distance in the sky
Angular Measurements
In astronomy, angular
measurements are made
to determine locations of
celestial objects from
their position
Degree = a circle’s divided
into 360 degrees
See diagram at right to
determine the
approximate size of 1, 5,
10, and 15 degrees
Moon and sun are only 0.5 °
(stays constant--if it looks
bigger...it’s an illusion)
Defining a day.
(These terms are not set times, just positions)
Your position, depending on where the earth is in it’s orbit with respect
to other planets in their orbit, will allow you to see the planets at night,
or not.
Sidereal Day vs. Solar Day
Sidereal Day
The time it takes a star to line
back up with Earth
23 hours, 56 minutes, 4
seconds (~4 minutes
shorter than a 24 hr day)
Therefore stars rise
~ 4 minutes earlier each
night
Mean Solar Day
=24 hrs (avg. time for Sun to
return to same position)
What is the ecliptic?
Definition #1
the daily path
of the sun
in the sky
Definition #2
the path of
the sun on
the celestial
sphere over
the course
of the year
The path of the
sun changes
over the course
of the year
Motion and Positioning
of the planets in the
sky.
You can only see the
planets that are on the
night side of the Earth
Jupiter
Mars
Venus
moon
Mercury
Notice that the planets lie along the ecliptic because the solar system is in the same plane (a
flat disc)
Imagine the sun here below horizon
in west
Motion of the Planets
The Sun, Moon and planets can always be found in a Zodiac
Constellation on the ecliptic.
Planets "wander" across the celestial sphere and through the
stars in two ways:
1. Direct Motion - normal eastward movement of planets
2. Retrograde Motion - occasional westward movement of planets
The Motion of the Planets
Mercury appears at most ~28°
from the sun.
It can occasionally be seen
shortly after sunset in the
west or before sunrise in the
east.
Venus appears at most
~46° from the sun.
It can occasionally be seen
for at most a few hours after
sunset in the west or before
sunrise in the east.
Planets that are NOT visible
Planets may not
be visible at night
because of where
they might be in
their orbits.
What if Jupiter
were here?
Retrograde Motion
Retrograde motion occurs
when an inner planet passes by
an outside planet and the
outside planet appears to go
“backwards” in the sky for a
few weeks (this doesn’t happen
in a night)
Click for Animation
The movement is apparent—
Mars does NOT really move
like that in space. The position
of mars shifts against the
backdrop of distant stars.
The positions of the stars change over the course of the
year because the Earth orbits the Sun…
Off-season
constellations are “up”
during the daytime
Notice that between
June and August, the
Sun is directly “in”
Gemini
Notice that June to
August, Sagittarius is
prime viewing during
summer nights
Phases of Inner Planets
The inner planets go through phases
similar to the Moon. Just as with the
Moon, it is the positioning between the
planet, the Earth, and the Sun that
determines how much of the
illuminated portion we see, and hence,
the phase. Figure 4 shows the phases of
Venus.
One big difference between the phases
of the inner planets and the phases of
the Moon is that the angular size of the
planet changes with the phase. As you
can see from Figure 4, the full Venus
appears much smaller than a crescent
Venus because a crescent Venus is
much closer to us. This means Venus is
actually at its brightest when it is in a
crescent phase.
Precession
At left, gravity is pulling on a slanted top.
Wobbling around the vertical.
The Sun’s gravity is doing the same to Earth.
The resulting “wobbling” of Earth’s axis of rotation around the
vertical w.r.t. the Ecliptic takes about 26,000 years and is
called precession.
Precession
As a result of precession, the celestial north
pole follows a circular pattern on the sky,
once every 26,000 years.
It will be closest to
Polaris ~ A.D. 2100.
There is nothing
peculiar about
Polaris at all (neither
particularly bright nor
nearby etc.)
~ 12,000 years from
now, it will be close
to Vega in the
constellation Lyra.
It moves 1 degree every
72 years
The Seasons
Earth’s axis
of rotation is
inclined to
its orbital
plane by
23.5°, which
causes the
seasons.
The Seasons
The Seasons are a result of
how direct the sunlight is
that strikes Earth.
Light from
the sun
They are not related to Earth’s distance from the sun. In fact,
Earth is slightly closer to the sun in (northern-hemisphere)
winter than in summer.
The more direct light is more intense and provides
more energy or heat.
The Seasons
Northern summer =
more direct light, sun
higher in sky, daytime
sunlight is longer
Northern winter = less
direct light, sun lower in
sky, daytime sunlight is
shorter