29:52 Characteristics and Origins of the Solar System
August 31, 2001
The Solar System in the Night Sky
(Part 2)
→ Initial
pleasantries…watch the position and appearance of the moon over the weekend.
The image for the
day: On Wednesday we looked at the image of the Sun. Let’s check it today and see if there are
any changes
Ø
http://sohowww.nascom.nasa.gov
Ø
Next, we can see the
difference in appearance of the Sun in visible light and ultraviolet light.
Last
time I began discussing seasonal changes in the appearance of the sky, These are:
·
Different constellations
are visible at different times of year.
·
As we go into winter,
the Sun rises further to the south, and is lower at its maximum altitude angle
or elevation angle.
·
It gets colder.
What is happening here?
The Explanation, is that the Earth is orbiting the Sun, and that its
axis is tilted 23.5 degrees with respect to the plane of its orbit. See
Figure 1.6 of textbook. The fancy term for this angle is the obliquity
of the ecliptic and it has a value of 23 degrees 27 arcminutes (23.45
degrees).
>diagram on blackboard showing
show obliquity of Earth's axis can affect sky positions of Sun.
·
The varying angle
between Sun-Observer at Noon-Pole Star is also responsible for temperature
changes associated with seasons, as well as fact that northern hemisphere and
southern hemisphere winter and summer are interchanged.
·
Seasonal changes in constellations
are due to orbital motion of Earth around Sun. We see the Sun "projected
against" different stars at different times of year.
>Look at SC1 constellation chart
showing dates of Sun's position.
>Look at SOHO web page showing
Sun moving against background stars.
(3) How Long is
the Day?
Would seem to be a stupid question.
24 hours=86400 seconds. But what do we mean by a day?
What is 24 hours is
actually more precisely known as the mean solar day. It is the average
time during the year between
successive transits of the Sun.
We could also define the day by the
time between successive transits of a star, say Vega which is nice and bright
these evenings. This is called the sidereal day. Strangely enough, it is
almost the same as the mean solar day, but not quite. (Nota Bene:
this material is discussed on p 63ff of your book).
The sidereal day is
23hours, 56minutes, 4seconds long.
>Question for the august
assembly: Why are the two days different?
(4) The Year
The year is the length of time it
takes the Earth to complete its orbit around the Sun. The official term for
this is the sidereal year, and could be measured by the time it took for
the Sun to start out at
one place in the sky
(defined relative to the background stars), move completely around the sky, and
return to the same place. The value of the sidereal year is 3.1558 X 10**7
seconds.
(5) The Year and
the Day are not commensurate numbers.
Two entirely different physical
periods lie behind the astronomical definitions of the day and the year. The
day (precisely the sidereal day) is the time it takes the planet Earth to turn
on its axis like a spinning top. The year is the time it takes to complete its
circuit around the Sun, like a racecar running around a track.
There is no physical reason for the
year to be an integral number of days, and it isn't. In fact, one year =
365.2564 solar days. The little extra 0.2564 days is responsible for annoyances
such as leap years, intercalcary days, the discrepancy between the Julian and
Gregorian calendars, etc.
Every high civilization, from Old
Kingdom Egypt to Classical Mayan, was aware of this discrepancy, although they
naturally did not have the correct physical explanation for it.
® Let’s think about the significance of this fact. 1 year is 365.2564 mean solar days. To simplify things a bit, let’s assume that we start the year at noon on January 1, noon being when the Sun is on the meridian. We then count 365 days, and exactly 365 mean solar days later, as the Sun crosses the Meridian, we celebrate the start of the new year. The trouble is, the Sun has not returned to the same place relative to the stars. It is still 0.25 days away from that point. After the next year, it is half a day, and the “error” accumulates at the rate of 0.25 days per year.
®A calendar based on the rotation of the Earth (number of days) and one based on the position of the Sun in the sky (seasons or years) will get out of synch rapidly (who wants snow in July?).
There are two approaches: keep inserting fudge factors from time to time to keep the two in synch (the European approach) or say the heck with it and not care if there is a mismatch between the month number and the season (Mesoamerican Indian approach).
6. Lines on the Sky.
There are two important lines on the sky. Celestial Equator= expand equator of Earth out to the celestial sphere. 90 degrees from the celestial equator is the north celestial pole, 90 degrees south is south celestial pole.
The ecliptic is the line across the sky made by the Sun in the course of the year.
It is inclined with respect to the celestial equator.
̃ Drawing
on blackboard and chalk sphere
̃ Look
at Figures 1.2 and 1.6 in textbook
̃ Look
at SC1 starchart for both of these.
These two
lines on the sky (actually great circles on the celestial sphere)
intersect at two points. One of these is the Vernal Equinox and the
other is the Autumnal Equinox.
7. Celestial Coordinate
System
We need a coordinate system to describe the positions of objects in the sky. Corresponding to longitude on Earth is Right Ascension, with an origin at the vernal equinox.
̃ Look at SC1.
Corresponding to latitude on Earth is Declination.
̃ Look at SC1.
Example: Right now the celestial coordinates of Uranus are RA= 21hours 21minutes, DEC = -16.25 degrees. Find where it is on the star chart.
8. Precession of Earth
̃ Description of physical phenomenon of precession.
̃ Laboratory demonstration of precession.
The rotation axis of the Earth precesses with a period of 26,000 years. There are two consequences of this.
A. The pole star changes with time: 3000BC, Thuban. 14000AD Vega.
̃̃ See Figure 1.11
B. Right Ascension and Declination slowly change with time.
9. Further Significance of
the Ecliptic
The ecliptic represents the intersection of a plane with the celestial sphere. This plane is approximately the orbital plane of all the major planets (but not all solar system objects).
̃ See Figure 1.5
̃ Look at Appendix 7 in text.
̃ Draw diagram showing orbital inclinations
Examples of inclinations are 7 degrees for Mercury, 3.4 degrees for Venus, all others smaller.
The plane of the ecliptic is very close to the plane
of the Sun’s equator. The major planets lie in the Sun’s equatorial plane. (A
big hint as to what was going on when the solar system formed).
10. Where do we see planets
in the night sky?
Relation relative to the Sun: different for planets interior to the orbit of the Earth and exterior.
œ For planets interior to the orbit of the Earth;
œ For planets outside the orbit of Earth,
· Draw orbit of planet
· Opposition and Superior conjunction.