Do you know?

What is an astronomer and what type of work do they do?

Astronomers observe and define the night sky.  They observe the positions, brightness, spectra etc. of the stellar objects such as comets, asteroids, planets, stars, nebulae or galaxies.  They may also classify and name them in a systematic way. 

An astronomer might be good at stargazing and recording but poor in mathematics, such as Tycho.  On the other hand, an astronomer might be skillful in mathematics and analyzing but poor in stargazing such as Kepler. 

If an astronomer has a strong physics background, he/she might be able to use the law of physics to construct theories or models to describe the motion of the planets or to understand the formation of stars, galaxies or even the universe.  In this case, we may better call them astrophysicist (such as Chandrasekhar) or even cosmologist (such as Hawking or Rees). 

An astronomer may also be an elegant photographer, technician or engineer without much physics background.  They may be very skillful in grinding lens, aligning a telescope, positioning a star or taking great star photos. 

Nowadays, an astronomer can be a sophisticated computer programmer.  They write large program and use computer to simulate phenomena that might never be observed, such as the collision of two galaxies or the formation of black holes.

An astronomer may also be an educator.  They simply want to share with the others, especially the young generation, the joyfulness of observing and knowing this comprehensible universe.

How does the daily path of the Sun across the sky change with the seasons?

As there is a tilt of 23.5° between the equatorial plane and the ecliptic plane as shown above, the Sun’s daily path (due to the Earth’s rotation) across the sky  varies with the seasons.  

On the first day of spring (21/3) and on the first day of fall (21/9), when the Sun is at one of the equinoxes, the Sun rises precisely in the east and sets precisely in the west.  During summer in the northern hemisphere, the Sun rises in the northeast and sets in the northwest.  The Sun reaches its northernmost position at the summer solstice.  In winter in the northern hemisphere, the Sun rises in the southeast and sets in the southwest.  It reaches its southernmost position in the winter solstice.

How did the Michelson-Morley experiment show that the velocity of an observer doesn’t affect the measured speed of light?

During the end of the 19^th century, scientists generally believed that light travels with a speed of 3 ´ 10⁸ m s^-1 with respect to a fixed ether[1] frame.  If so, an observer moving through ether with a speed v would measure a speed c' for a light beam, where c' = c + v, i.e., the measured speed of light depends on the observer's velocity.  It was this prediction that the Michelson-Morley experiment was designed to test in 1887.

A simplified version of the Michelson-Morley experiment is shown below.

Beam 1 and beam 2 are coherent (division of amplitude), so they would interfere at the telescope T.

Whether the interference is constructive or destructive can arise from two causes:

(1)       the path difference between beams 1 and 2 (i.e., the different path lengths l₁ and l₂) and

(2)       the different speeds of light with respect to the instrument (observer) because of the ether wind v.

The second cause is more crucial one.

The time for beam 1 to travel from M to M₁ and back is:

(The 1^st term is the upstream speed and the 2^nd term is the downstream speed.)

Beam 2 travels in cross-stream path through the ether.  The transit time is given by:  

Thus the difference in transit time for the two beams is:  

If the whole set of instrument is rotated through 90° (i.e., l₁ and l₂ are interchanged), the same analysis as above can give the difference in transit time as:  

Hence, if the measured speed of light does depend on the velocity of the observer, the rotation of 90° would change the time difference by:  

Thus, one can see that the rotation changes the phase difference between beams 1 and 2.  This results in a shift in fringe pattern observed.

Let DN be the number of fringes moving past the crosshairs of the telescope and T be the period of the light wave being used.

In the 1887 experiment, l₁ + l₂ = 22 m, l = 550 nm and v/c = 10^-4, this will give an expected fringe shift DN = 0.4.  However, observations were made day and night and during all seasons of the year, but the expected fringe shift was not observed.  This null result implies that the measured speed of light does not depend on the motion of the observer.  

Why does time pass more slowly for moving observers?

This can be illustrated by the simple experiment below.  Imagine a passenger sitting on a train that moves with uniform velocity v with respect to the ground.  The experiment will consist of turning on a flashlight aimed at a mirror directly above on the ceiling and measuring the time it takes the light to travel up and be reflected back down to its starting point. 

The passenger, who has a wrist watch, say, sees the light ray follow a strictly vertical path from A to B to C and times the event by his clock (watch).  This is a proper time interval, measured by a single clock at one place, the departure and arrival of the light ray occurring at the same place in the passenger's (S') frame.

Another observer, fixed to the ground (S) frame, sees the train and passenger move to the right during this interval.  He will measure the time interval from the readings on two stationary clocks, one at the position the experiment began (turning-on of flashlight) and a second at the position the experiment ended (arrival of light to flashlight).  Hence, he compares the reading of one moving clock (the passenger's watch) to the readings on two stationary clocks.  For the S-observer, the light ray follows the oblique path as shown below. 

Thus, the observer on the ground measures the light to travel a greater distance than does the passenger.  Because the speed of light is the same in both frames (explained in question 1), the ground observer sees more time elapse between the departure and the return of the ray of light than does the passenger.  He concludes that the passenger’s clock runs slow.

Why do the giant planets have many more moons than the terrestrials?

At the very beginning, the giant planets such as Jupiter formed from massive, rocky core that drew gas onto it by huge gravitational attraction.  This gas formed a so-called “Jovian nebula” around the core.  The dust grains in the outer part of this nebula could have accreted to form smaller solid bodies.  The solid bodies grew to form the satellites.  On the other hand, the terrestrials were not massive enough to form such nebula.  This explain why the giant planets have many more moons.

Why are photographs of reflection nebulae typically blue?

The reflection nebulae are regions of low-density fine dust grains.  When visible light from a nearby star encounters those interstellar dust grains, blue light will be much more easily scattered by the dust grains in different directions.  Hence reflection nebulae are typically blue.  The longer wavelength red light is only slightly scattered and will more or less pass straight through the dust grains.  This explains why during sunset, the sun looks red.

How do we know that the phases of the Moon are not due to the Moon moving in the Earth’s shadow?

There are two reasons:

(a)    We can see the different phases of the Moon almost over the entire month.  If the phases of the Moon are due to the Moon moving in the Earth’s shadow, we can only see the Moon phases for a few days only.

(b)    One phase of the Moon is as shown:

The Moon phase shown above will never occur if the phases of the Moon are due to the Moon moving in the Earth’s shadow.  When the Moon enters the Earth’s shadow, Moon eclipse occurs and the phase of the Moon must be either in the form of: