Physics 1 - Astronomy Study Guide

The reason why you can’t see stars in the day is because of Atmospheric Scattering. The sky is blue because air molecules scatter blue light more than red light. Blue light has a shorter wavelength than red light. On sunsets and sunrises, the sky turns orange reddish. This is because light has to go through more atmosphere and encounters more dust closer to the ground, causing the scattering of more red light.

Dynamic Range is contrast between brightness (contrast between the darkest and lightest spots). A good planetarium comes close to displaying accurate dynamic range, but it does not have the dynamic range of a dark night sky.
Light Pollutionmakes black areas of the sky look grey. This is caused by scattering from light bulbs – light bounces off of air, dust, or water droplets and goes into the human eye. This results in our incapability of seeing the dimmest stars. When observing the night sky, it’s best to be at least 100 miles away from any big city.
In LA, only about 2 dozen stars are visible (in a clear dark night sky, you can see about 6,000 stars). Faint stars and nebulas are washed away due to light pollution. Galaxies look small, cloudy, and faint (you can usually only see the core of a galaxy with the naked eye). The closest galaxy to the Milky Way is the Andromeda galaxy – our sister galaxy that’s on a collision course with ours. If the Andromeda galaxy were brighter, it would be like 5 full moons in line with each other.
Mount Wilson was an observatory in Los Angeles that held the largest telescope in the early 20th century. However, it’s no longer in use due to the massive light pollution. It’s known for its galaxy research and for Hubble discoveries. (Griffith Observatory is not meant for science, it’s for public viewing.
What can you see from the city? Very bright objects, such as planets. Saturn is insanely bright and can be seen everywhere in the world. Saturn looks like a point without a telescope. When doing visual astronomy with your naked eyes, it takes about 30 minutes for your eyes to adjust to the dark and for your pupils to fully dilate. When pupils are 100% widened, you receive about 100 times more light. Pupils contract in a split second as a safety precaution, preventing your retina from burning.
Objects that are barely rising are harder to see because of the amount of atmosphere the light must go through. Turbulent Air, violent irregularities in the air, cause planets and other objects in the sky to look blurry and out of focus.
The North Star moves in a very small circle, even though it looks like it’s fixed at one point. Its proper name is Polaris (Alpha UrsaeMinoris. It looks like the brightest star in the sky because there are no stars close to it, making it look bright in contrast. However, the brightest star in the sky is Sirius A (Alpha CanisMajoris). The North Star is located at the North Celestial Pole.
Only the Northern Hemisphere can observe the North Celestial Pole. You can easily find Polaris by finding the big dipper. The two stars at the end of the big dipper point to Polaris:

In the southern hemisphere, the South Celestial Pole is just a dark patch of sky with no stars.
Only the Southern Hemisphere can observe the South Celestial Pole. The Southern Cross is a constellation that points to the empty patch of sky where the South Celestial Pole is located.
Our modern astronomy is directly derived from the ancient Greeks. They figured out that the sky spun (believing that the Earth was fixed). Hipparchus, a famous Greek astronomer, mapped out the sky and indicated where all the stars were located. He created a ranking system for brightness, making the brightest stars 1stmagnitude and the dimmest 6th magnitude. Now there’s a formula that fits Hipparchus’s scale, where the brightest stars have about 1 magnitude (or less) and the dimmest stars have 6 or more magnitude. Since small numbers mean brighter objects, some celestial bodies have negative values. The moon, for example, has a magnitude of -12.7, and the sun has a magnitude of about -27.
Why didn’t they use Polaris as a reference for magnitude? Polaris had ranking 2 and scientists tried to define Polaris to be 2.00 magnitude. This was a bad decision because Polaris is a variable star. Variable stars pulsate, growing brighter and getting dimmer. They chose another star for their reference, Vega.
Constellations have no scientific importance but are still used by astronomers. They’re useful in dividing the sky into arbitrary patches.
There are 88 constellations that originated in Greece, and a few constellations in the Southern hemisphere that were added later. Greeks named a handful of stars, but the majority of stars have Arabic names. “Altair” is Arabic; Al = the; Altair = “the bird.”
There are different naming systems in constellations. They name the brightest star “Alpha” (α).Aldebaranα Tau is the brightest star in the Taurus constellation. The order of brightest is alpha, beta, gamma, delta, etc.
The big dipper isn’t a constellation, it’s an asterism, a pattern of stars that’s not officially recognized by the international scientific community. The big dipper is a part of the Ursa Major constellation.
The sky has depth, but from earth, it looks like a big flat hemisphere. We treat the sky as half a globe and grid it off like latitude and longitude. This is the Horizontal Coordinate System.
The horizon is like the equator and the point straight above is called the zenith. The point straight down is called the nadir.
Latitude = altitude; longitude = lines of azimuth. There’s negative altitude, but it’s below the ground. 0 degrees azimuth is due north, to the right is 10 degrees, 20 degrees, and so on. (Thumbs are 2 degrees and a clenched fist is approx. 10 degrees).
Degrees, Minutes, Seconds: 1/60 of a degree is 1 minute (or arcminute). 1/60 of a minute is 1 second (or arcsecond).
23° 54’ 22.175’’ = 23° + 54/60 + 22.175/60 = 23.90615972
57.2° .2° = 1/5° = 1/5 (60) = 12’ =57° 12’

The other type of coordinate system is the Equatorial Coordinate System. This grid moves along with the stars.
Latitude = declination; longitude = ascension. Ascension does not use degrees, it uses hours. It starts at 0h and goes up to 23h, each segment is 15 degrees apart. The 0 line of ascension is where the ecliptic crosses the equator; the spring crossing.
The Ecliptic is the line the sun travels on; the path it takes. Most planets follow the ecliptic because the solar system is almost in the same plane. Certain points on the ecliptic mark the beginning of seasons when the sun passes through them.
On this grid, the stars are fixed but the planets, the moon, and the sun travel against the stars (Greeks dubbed them wanderers).
Crescent moons are the best to look at during the day. The day-night divider is called the terminator (the division between the illuminated and dark parts of the moon). When you look near the terminator you see a lot of surface detail. Full moons are bad for observation because they do not cast a shadow, thus revealing no surface detail.
A rare occurrence is taking place at this time: Mars is rising in the east with Saturn right behind it. Mars is very bright and stays bright all night. Earth’s orbit is faster than Mars’s orbit – and Mars is sometimes too small to see any detail. However, with this geometry and opposition(an earth-centric term meaning it’s opposite the sun), you can see Mars with much more detail:

When any 3 or more things in space are collinear, it’s a Syzygy. Opposition is a special case of syzygy.
Each year, sun goes right-right-right-left on the ecliptic. The jump left is caused by leap years. The idea is that there are 365 and ¼ days in a year. Each year, we fall behind, thus the leap year recovers that lost day.
Each constellation has boundary lines. Out of the 88 constellations, the sun crosses 12 of them – the astrological signs.
Seasons. The first day of spring is when the sun crosses the celestial equator going north; the first day of summer is when the sun is at the peak of its travel (the most northern part of the ecliptic); the first day of autumn is when the sun crosses the celestial equator going south; and the first day of winter is when the sun is at the most southern part of the ecliptic.
The season are defined (astronomically) as follows: Around June 20th, the sun is at the northern point of the ecliptic and then heads south; Sept. 21st, the sun crosses the celestial equator going south; Around Dec. 19, the sun reaches the southern point of the ecliptic and then goes north; March 20th, the sun crosses the celestial equator going north.
The first day of spring is called the Spring Equinox (or Vernal equinox in Latin). Equi = equal; nox = night. During the equinox, day and night are equal in duration. The first day of autumn is called the Autumnal Equinox. The first day of summer is called the Summer Solstice, and the first day of winter is called the Winter Solstice. Sol = son; stace = still.
The days that mark the beginning of a season change over time due to a phenomenon called Precession. This won’t make any dramatic effects in a couple of years or in a decade, but in 11,000 years, January will be the beginning of summer and June the beginning of winter. It’s a slow process, every year the seasons change by 20-22 minutes. In a decade, it’s a day shift.
June has the longest day of the year but is not the hottest day of the year; December has the shortest days but February is the coldest month. This is because water stores a lot of heat, and it takes time for them to cool off, causing lag time.

There are no seasons at the equator. When there are it’s because air that traveled from somewhere else. In the North Pole, the sun almost never sets. It sets on the autumnal equinox and rises by March.
The angle of earth’s axis/tilt is 23.5 degrees.

Eclipses:

It looks like we should get an eclipse every month, but we don’t because the sun and moon’s orbit are not aligned. Also, the shadow cast is small; so if the moon is just slightly in or out of the plane then it will not be an eclipse. There are more lunar eclipses because the Earth has a bigger shadow. In a billion years from now, eclipses will be impossible – the moon would have gone away by then.
Other things pass in front of the sun: Venus and Mercury. It’s not an eclipse if it doesn’t pass the whole sun, and Venus is so small that you won’t notice it. These “mini-eclipses” are called transits.

Telescopes were built in the 1500s for military purposes. Galileo was the first to use telescopes for astronomy.
Binoculars also magnify – 7x30 binoculars means it magnifies x7 and the lens are 30cm wide.
Telescopes not only magnify, but also brighten. A fully dilated pupil is about 7mm, but a telescope lens is about 30mm, increasing the light you’re getting by 16 times.
When air is turbulent, stars twinkle. Turbulence is also caused by the warm ground; the best time to observe and do astronomy is about 3-4 in the morning, when the ground has cooled off.
Astronomers use pictures to do science since it’s better than just looking through an eyepiece. Modern electronic sensors are more sensitive than your retinas, and can capture color better than the human eye can.
When you’re looking at the sky, you’re observing things in the past. The Andromeda galaxy, for example, is 2 million light years away, meaning that you’re looking at it as it was 2 million years ago.
Astronomical Distance is difficult to understand and requires a sense of perspective.
Diameter of the Earth 13,000 km (8,000 mi)
Distance to the Moon 384,000 km 30 Earth diameters Just a little over a light second (distance light travels in a second)
Distance to Sun 150,000,000 km 1 Astronomical Unit (AU)