Astronomy and Space Science I. Dr. Hoi-Fung Chau and Dr. Alex Tat-Sang Choy Jointly Organized by The Space Museum HKU Physics Department Co-organized by Science Education Section, EMB. Astronomy and Space Science. Astronomy Basics Length, time, angles Celestial sphere, star maps
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Dr. Hoi-Fung Chau
Dr. Alex Tat-Sang Choy
Jointly Organized by
The Space Museum
HKU Physics Department
Science Education Section, EMB
Andromeda Galaxy (M31)
180’ x 63’.
Orion Nebula (M42), 85’ x 60’.
globular star cluster, 12’
Pleiades, open star cluster, 180’.
M4, globular star cluster, 36’
1.4’ x 1’.More Examples
Io, Jovian satellite, 1”.
Polaris A’s apparent size = 0.002”.
Polaris A to Polaris Ab is 0.2”;
Polaris A toPolaris B is 20”;
Polaris A to Dubhe ≈ 30°.
Hubble Deep Field, ≈ 1.5’.
Vernal equinox(春分), autumnal equinox(秋分) are the points at which the Sun passes
the celestial equator, while summer solstice(夏至) and winter solstice(冬至) are the
northern and southern extreme points of the ecliptic (黃道).
Figure of showing the Earth around the Sun in the ecliptic plane.
Planets usually moves on the celestial sphere from east to west (prograde motion)
near the ecliptic; while sometimes moves from west to east (retrograde motion).
Motion of Mars in 2003 and 2005. Time step=10 days.
Pictures from NASA.
Note: nebulae are usually in spiral arms.
Milky Way Galaxy (2-4x109 stars)
Note: globular clusters (105-106 stars) orbit the galactic core as satellites.
(100 superclusters shown)
Local Group (30+ galaxies)
* visible ≠ whole
but not visible has no physical relevance.
(100 groups/clusters of galaxies)
A: The IAU divides the celestial sphere into 88 constellations (regions) with precise boundaries (yellow dashed lines in the figure).
Each star belongs to exactlyone constellation.
The term “constellation” is also less formally used to describe a group of star visibly related to each other in a pattern, such as those connected by green lines in the figure. However, in such a scheme, some stars such as Sirrah in Andromeda, may be considered as both the head of Andromeda or part of the Square of Pegasus. Also, stars not connected by patterns still need to be assigned a constellation.
A: As shown on the graph: the longitude and latitudes of the Celestial sphere are called RA (right ascension) and DEC (declination). DEC runs from +90° to -90°. RA runs from 0 to 24 hours. Each hour has 60 minutes, and each minute has 60 seconds, just like the clock. The RA of zenith of a fixed location increases by roughly 1 hour for every hour in time.
(Note: Do not confuse the minute with arc minute which is
1/60°, both measure angles.)
Refer to the previous figure, the light blue lines are RA and DEC lines.
A: The center of the celestial sphere is the observer. In other words, each observer has a celestial sphere.
The celestial sphere is a device used to represent the direction of celestial objects for observation. For example, someone in Beijing would see the Moon’s position a little differently from someone in Hong Kong, due to parallax of the observing locations. Therefore, it only make sense to have a different celestial sphere (and the objects on them) for each for observer. Another example is the satellite or space station, which, due to there close distance from Earth, depends greatly on the location of the observer. Also, if one were to observe from Mars, it would not make sense if the celestial sphere is centered on Earth!
Note however that in most situations, we are observing on the Earth and most objects are far away so it is convenient to set the center of the Earth as the center of the celestial sphere.
A: Yes, but for all purpose in this course, the change has no real effect.
A: The Sun’s position relative to Vernal Equinox is important for determining the seasons and the calendar. A major function of the calendar was for agriculture.
A: Precession is caused by the torque applied by the Sun, the Moon, and the planets. The torque is the result of the gravitational pull on Earth’s equatorial bulge.
More: The lower left picture explains the effect due to the Sun. The lower right picture shows the 26000 year periodprecession of the north celestial pole.
Pictures from Wikipedia.
A: A (solar) day is the duration for the Sun to pass the meridian twice.
A: Some events are:
Great opposition of Mars (near perihelion) (火星大衝) occurs every 15-16 years. The one in 2003 was the closest in 60,000 years, which the media made a big deal of. However, as shown on the graph, the other great oppositions such as the 1988 one are not much further away.
Note: since great opposition occurs near perihelion, when Mars is the hottest, planet-wide dust storms could occur, so observe early.
Picture: C.F. Chapin,
A: See figure.
Image showing Aristotle’s model.
A: Ptolemy was approximating Kepler’s law, without knowing it.
Comparing the elliptical orbit of Mars (red) to a circle (blue).
A: Great care for accuracy, a whole lifetime of pursuit, and a lot of support.
Left: The sight’s aligned horizontally if the star can be seen just on the CBGF edge and ADHE edge at the same time. The vertical alignment can be found similarly. For solar alignment, sunlight is allowing to pass thru the hole in the front and fall on a circle drawn on the ABCD plate.
Image showing Tycho’s sight device.
Image showing drawing
of a very large device
(three meter diameter) of Tycho
A: No. But Galileo did designed and made his own telescopes, and improved on
them.He was ahead of others by a few months in telescope quality, enough for
him to claim most of the discoveries.
Galileo’s telescopes are quite unimpressive by today’s standard, with 0.5-1 inch effective objective aperture, about 15-20x power, and a very narrow (15’) field of view, not to mention significant aberrations. But they were the best at the time.
A: He was found guilty in his trial and sentenced to jail for life. However, his treatment was closer to house arrest. He worked and published during this time.
Some ideas Galileo held, such as the Earth moves around the Sun, the celestial bodies are not perfect, the Bible was not meant to teach science, etc., were considered heresy at the time. A less fortunate astronomer named Giordano Bruno was burned at the stake. To understand why Galileo was treated leniently, perhaps one should understand that Galileo was well known not only to those who practice science, but to influential people of the society and even to the Church. He made many discoveries such as the law of motion, measured gravity, invented a thermometer, studied the pendulum, etc. The physics taught at the time stress qualitative arguments, Galileo however believed in the importance of mathematics and experiments. He was thus called the “father of modern science”. What made him stand out from other scientist of his time, was the skill of mixing of theory and practice.Galileo was also very successful in getting supports from many people. Although there were people who refused to even look though the telescopes, Galileo succeeded in introducing the telescopes to many nobles and military officials who quickly understood the practical and military applications of the telescope.
A: No. Models, such as Tycho’s model, which require the Venus and Mercury to revolve around the Sun give the correct phase of Venus.
A: Definition by theInternational Astronomical Union (IAU) in 2006:
(1) A planetis a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so
that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighborhood around its orbit.
Since some recently found “minor planets” are similar in size or even bigger (Eris) than Pluto, there was a need for redefinition. The new definition is based on planetary formation theory that, given enough time, a large enough object would be able to collide with or scatter away objects and dominate its orbit. The redefinition has been criticized and remains controversial. Note also that the line between (2) and (3) is left for later meetings. For many small object, the hydrostatic equilibrium condition (b) is not easy to test.
A: First by using wired micrometer eyepiece, then by measuring photographic plate.
More: The wired can be moved to match the star’s position. In some other eyepieces, a patterned glass is placed at the focus for reading out data. Angles can be measured from a photographic plate using the focal length and the lengths measured on the plate.
Image showing the view in a cross haired eyepiece.
A: Ancient Chinese astronomers developed sophisticated tools to observe the positions of celestial objects. Unfortunate their work did not affect the western astronomy development much.
More: As an example, the drawing on the right shows an invention in the Song Dynasty. The main instruments (red, blue, and yellow) are driven by water-powered gear systems to simulate Earth’s rotation and tell time automatically.
Source: HK Science Museum,
A: Q1: No. Q2: Yes.
A: Easier to compute, correct orbital radii, predicts stellar parallax.
A: It has been decreasing since history.
Here are some paradigm shifts:
A: Here are some of them:
Sources of pictures: Pictures are obtained from the following sources unless given next to the pictures.