Transcript Day 1 - Ch 1

Physics 101 Astronomy
• Dr. Brian M. Davies
• Office: 532 Currens Hall, 298-1307
• Office Hours are M W F 11-noon and
Tues 10-11:30 a.m.
• E-mail: [email protected]
• http://faculty.wiu.edu/BM-Davies/
• See the webpage for notes & syllabus
“Discovering the
Essential
Universe", by
Neil F. Comins,
6th ed.
Ch. 1:
Scales of
the Universe
What can we see in the visible sky?
• Humans can see about 6000 stars in the
night sky (with good vision and a very dark
clear night).
• Some of these form patterns called asterisms.
• These have been grouped into constellations
(88 in the current system). Most have old
names from mythology; those in the southern
hemisphere have Western names.
The Constellation Orion, as seen in the sky and as imagined.
Why constellations?
• Why do people invent constellations in
the night sky?
• Before indoor lighting, many people
spent their evenings under the stars,
with plenty of time to talk and observe.
Constellations are invented for
• Story-telling, mythology, ritual
• Culture and religion
• Navigation and timekeeping
• Mapping the sky; modern astronomy
uses the constellations to label areas
that are delimited by boundaries.
Navigation
• The pole star, Polaris, can be used to
determine the direction toward north.
• The star Hydra may have been used by
Minoan sailors to get East-West directions
(2400 BC).
• Polynesian sailors used celestial navigation.
• Until recently, sailors used celestial
measurements for navigation of ships.
Mapping the sky: Constellations can be used
to divide up the sky into regions.
For example, Orion can be used two ways:
as an asterism.
as a region of the sky.
The Celestial Sphere
• The distance to the stars is not evident
to our eyes, and they appear to be at
the same distance.
• If we think of this distance as a radius,
the stars appear to be on a sphere, with
us (on the Earth) at the center.
The Constellation Orion is actually three dimensional, but
appears to us as a group of points on the “celestial sphere”
For more images, see this link, which will be given in the
class notes for the day, available from my website. (link)
The Celestial Sphere appears to rotate around us at night.
But you know that it is the Earth that is rotating. (link)
To observers who think the earth is stationary,
The celestial sphere appears to be rotating.
The Northern Sky, in a time exposure,
shows the apparent motion of the northern part
of the celestial sphere around the Pole star, Polaris.
Right Ascension and Declination are used to indicate
positions on the celestial sphere. They correspond to
latitude and longitude on the surface of the Earth.
The celestial sphere is oriented with respect
to the earth, with poles and an equator.
On the celestial sphere we use Declination
like we use Latitude on the Earth.
On the celestial sphere we use Right Ascension
like we use Longitude on the Earth, but
measured in hours, minutes, and seconds.
For an observer on the ground,
directions are defined in this figure.
The observer on the ground can also use angles
for altitude (Alt), the angle up from the horizon, and
azimuth (Azm), which is a heading (N, E, S, W etc.).
For more detail on Right Ascension and Declination, see this.
Stars are (almost) fixed on the celestial sphere and the
Earth rotates inside this sphere. It only looks like the celestial
sphere is rotating from our perspective if we are on the Earth.
Question ?
• To us, which way does the celestial
sphere appear to be rotating?
• Hint, think about the motion of the moon
and the sun during the day or night.
• Now, if the celestial sphere is actually
not moving, and the Earth is rotating,
which way does the Earth rotate?
Apparent rotation of the celestial sphere
is due to the rotation of the Earth.
The Earth is rotating around an axis that
goes from pole to pole through a center.
Eventually, each day, the Sun sets in the
west. If we suppose the Sun is the
center of the solar system, it is fixed, so:
Each point on the surface of the Earth is
going east all the time (except the poles).
View of sunset from 100,000 km
above the Earth, as it rotates.
• I got the latitude and longitude from a web
site: www.lat-long.com
• Then I put them into the Earth-Moon viewer:
http://www.fourmilab.ch/earthview/vplanet.html
• Start the view 100,000 km above Macomb, IL
at midnight UTC, June 7 (in local time, 7:00
p.m., June 6) and get a picture each hour
after that as the Earth rotates. See the result
in the following slides:
Seen from far above the North Pole, the Earth
appears to be rotating counterclockwise (CCW).
Sun
If the Sun is directly above point A, then it
is local noon there, and in 24 hours it will
again be noon at that location on the Earth.
The Earth is also in orbit around the Sun,
taking 365.25 days to revolve once around.
This orbital
motion is also
CCW if viewed
from above the
north pole.
In 24 hours, which is called the solar day,
the Earth must rotate more than 360 degrees!
Solar vs. Sidereal Day
• The solar day is 24 hours long, by
definition, but Earth actually rotates
through an angle of 360.986o in order to
be aligned with the Sun. This is due to the
orbital motion of the Earth, which means
that the Earth has to rotate an additional
360o/365 or 0.986o per solar day.
Solar vs. Sidereal Day
• The sidereal day is, by definition, the
times it takes the Earth to rotate around
and come back into alignment with the
stars. This is a rotation of exactly 360o
and this takes 3.9 minutes less than 24
hours.
•
1 sidereal day = 0.9973 solar days.
Why is this relevant ?
• The difference between the solar day
and the sidereal day means that the
Sun and the stars appear to be going
around the Earth at different rates.
• The Sun goes around in 24 hours.
• Stars go around in 23 hours, 56.1 min.
• So the Sun is not in the same place on
the celestial sphere day after day.
The Zodiac is the set of constellations that the Sun
appears to go through during the course of one year.
The Ecliptic is the path of the Sun on the celestial sphere,
which is tilted with respect to the celestial equator,
due to the tilt of the Earth’s axis with respect to our orbit.
The axis of the Earth is not perpendicular to
the plane of the orbit of the Earth around the Sun.
The Earth is tilted by 23.5o.
Seasons are due to the changing orientation of the
Earth and Sun, not because the
Earth is orbiting closer or further from the Sun.
Ch. 1, to be continued.
Things to do:
• Get the textbook. Access cards not
needed! A used copy should suffice.
• Read Ch. 1. Or look at my notes.
• Look at the syllabus and the dates.
• Mark your calendars for exam dates.
• Welcome back to WIU!