Is
this night or day?Page last updated January 26, 2005 11:48 AM
Astronomy full of cycles, such as the day, the year and the month. These are due to things like the rotation of the Earth, the orbit of Earth round sun, orbit of moon round Earth and orbit of planets round Sun. In this section we will try to understand these cycles.
I will be using computer software to demonstrate how the sky changes. Usually I will be using software called "Starry Night".
Goals here are
I'm doing this at an early stage in the course because I want you to watch the sky over the next few months
This is possibly the hardest part of course. It took civilization thousands of years to understand what is going on so you should not feel bad if it takes you a couple of weeks. There is a simple reason why this section is difficult. We are trying to understand the apparent motion of moving objects (such as planets) when we ourselves are also moving, albeit too slowly for our senses to detect directly.
Is
this night or day?
Most
of what we see in everyday life is scattered light; exceptions are electric
light bulbs, and the Sun which actually generate light rather than scatter it.
In astronomy it is particularly important to distinguish between objects that generate light (eg the Sun and the stars) and objects which scatter (or reflect) light, such as the Moon and the planets.
The
sky is bright during the daytime because of scattering of light in the Earth's
atmosphere.
If you look at a photograph of astronauts on the moon you see that the sky is dark, even though the Sun is casting shadows.
When there is less atmosphere above you the sky looks darker: look upwards out of an airplane window, and see that the sky is a dark blue. Clouds scatter light more so they are brighter than the blue sky.
We exist in a Universe with stars all around us. The only reason we don't see them in the daytime is that the sky around them is too bright. With some kinds of telescope we can see stars even in broad daylight.
The
Earth's atmosphere does not have a sharp edge. It tails off gradually, so that
the pressure is down to about half the sea level pressure at an altitude of 7
km. You can see a blue edge to the Earth in satellite pictures, such as
this picture of the MIR spacecraft.
At the summit of Mauna Kea you are above about 1/3 of the Earth's atmosphere.
Space shuttle orbits at about 100 miles, which is small compared with the diameter of the Earth which is 8000 miles. In human terms this is almost like being in a vacuum, but there are still traces of the Earth's atmosphere even at this altitude.
Computer demonstration of removal of Earth's atmosphere, using Starry Night software
Consider
the Earth in the middle of a 3-dimensional forest of stars. For this section we
can forget about the Sun, the Planets and the Moon, and ignore the rotation of
the Earth. If we stand in the desert or in the ocean we can see half of
the universe; our "horizon" defines what we can and cannot see.
The point directly above your head is called the "zenith"
Just by glancing at the sky we don't know the distance to stars (to measure distances to stars we need to make very careful observations with a telescope; we'll learn about those later.). What we see is what looks like a pattern on the inside of a sphere. We can make maps of the patterns of stars. From the surface of the earth we can never see more than half of the sky at a time, but we can piece together an atlas of the whole sky.
There are problems in making maps of large regions of the sky on a flat sheet; these problems are the same as those that face geographers making maps of the curved surface of the Earth.
The positions of stars relative to each other change only very slowly.
(Although the positions of the stars relative to the horizon change by the hour,
as we shall learn later)
Ancient
astronomers saw almost exactly the same patterns we do thousands of years ago.
They called these patterns constellations, and often linked them with
their mythology.
Note that the stars in one constellation not necessarily at the same distance
from the Earth, and are probably quite unrelated to each other in space.
The whole sky is divided into constellations, just as the whole USA is divided into states
Show constellations and constellation boundaries on "Starry night"
Only very bright stars have names. In the West these name are often derived from Arabic names. Most stars are referred to either by number or by their position in the sky.
Millions have numbers; billions more exist on photographs and have never been given names
Many people enjoy learning and recognizing constellations, but it is not necessary for this course.
In
describing the sky's appearance we need to discuss the angles between stars
We can talk about one star being 10 degrees from another. This is nothing to do with how many miles or light years they are apart.
We use degrees for large angles, for small angles we use arcminutes (1/60 of a degree) and arcseconds (1/60 of an arcminute)
The Sun and Moon both look to be about 1/2 degree across on the sky.
A penny held at arms length is about 1 degree (depending on your arm!)
The coordinate system that astronomers usually use to describe the position of a star is rather like latitude and longitude but I do not need you to understand how to use it. Like latitude and longitude, the coordinates are based on angles.
For this section we will continue to ignore the Sun, Moon and planets. The difference from the last section is that now we are considering the effects of the fact that the Earth rotates about its axis once per day.
Rotation
of the Earth means that stars appear to move. The motion can be seen with a time
exposure, or speeded up with a computer.
Starry night: zoom out, switch off atmosphere and watch rotation.
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If you look down onto the Earth from above the North Pole the Earth appears to rotate to counterclockwise. In other words it rotates towards the East. Stars therefore appear to move from East to West in the sky. |
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Foucault was a Frenchman in the 19th century who built a large pendulum with very low friction which could keep swinging all by itself for several hours. When you watch such a pendulum you will notice that the plane of its swing appears to rotate over a period of hours. This is easiest to understand if we imagine the pendulum at the North Pole. Once the pendulum is set swinging it stays swinging in the same direction in space, even as the Earth rotates underneath it. If the Earth was fixed and the sky actually rotating then the pendulum would not appear to rotate with respect to the Earth |
Before we go any farther we need to define the Celestial Poles and the Celestial Equator: Celestial poles are the points in the sky directly above and below the rotation axes of the Earth. Celestial equator is a line in the sky equidistant from the poles. The celestial equator divides the sky into Northern and Southern hemispheres. Polaris is very close to the North Celestial Pole. The "Belt" stars in Orion are very close to the Celestial Equator.
The latitude of a place on Earth describes its North-South position. Hawaii has a latitude about 20º North. Lines of latitude run from side to side on a map.
(If you confuse longitude and latitude remember "Latitude is Flatitude")
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The angle between the horizon and Polaris is equal to your latitude.
Photo of star trails again
Demonstrate changing location of Polaris using Starry Night software
If you stand at the Earth's north pole you will always see Polaris above you, (ignoring the brightness of the daytime sky) with the sky rotating around Polaris. The same stars are visible above your horizon all the time. No stars ever rise or set.
If you stand at the Equator, Polaris will be on the horizon, and all stars will rise and set. In principle you can see all the stars in the Universe at some time or another, although only half will be visible at any time. The celestial equator runs directly overhead from east to west.
If you stand at an intermediate northern latitude you will see Polaris at an angle above the horizon that is equal to your latitude (about 20 degrees in the case of Hawaii). Some constellations near Polaris are visible all through the night. They are called circumpolar. Some constellations near the south celestial pole are never visible. Most constellations can be seen some of the time as they rise in the East and set in the West
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It is (in principle) very easy for a navigator to determine his/her latitude on the Earth. All you need to do is measure the angle between Polaris and the horizon. If you can't see Polaris (in the southern hemisphere, for example) you can use other stars and make some calculations, but the principle is the same.
Longitude describes the East-West position of a place on the Earth's surface. London (actually Greenwich) is at longitude 0º, and Hawaii is at (roughly) longitude 157º west.
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At any time of night, the exact appearance of the sky depends on your longitude as well as your latitude. Stars transit at different times at different longitudes on Earth. The same star crosses the meridian about 10 minutes later in Honolulu than in Hilo and about 2 hours later than in Los Angeles Demonstration using Starry night |
It is more difficult to measure longitude than latitude. The problem is that two things affect when you see a star transit the meridian; the precise time of day, and the longitude. If you know one, you can calculate the other. If we want to determine the difference in longitude between two places (say Honolulu and Los Angeles) using the stars, we need to know the difference in times that a particular star transits. If we are in Hawaii, we therefore need to know the time in Los Angeles. Until the time of Captain Cook this was impossible, since there was no radio, and clocks were inaccurate. Only after the invention of the accurate chronometer by Harrison in the 18th century was it possible to measure longitude at sea with confidence.
I recommend the book "Longitude" by Dava Sobell (Penguin books 1995) for the story of Harrison. There is also an A&E 6-part TV series available on DVD.
Polynesian
navigators did not have clocks, so had to resort to other subtle clues for
determining longitude. These techniques have been reinvented by Nainoa Thompson
and colleagues, who sailed the "Hokulea" all around the Pacific.
Nowadays most people navigate by receiving signals from the Global Positioning Satellites (GPS). There are 24 special satellites in 12-hour orbits around the Earth. You can buy a receiver that will give your position to within 10 meters for about $100
Commercial site with good description of GPS receivers
Historically our measurement of time was always based on the rotation of the Earth. Now that we have very accurate atomic clocks we find that there are small variations in the rotation speed of the Earth, probably due mainly to changes in the motions of the molten iron in the earth's core. Astronomers keep track of the rotation of the Earth and decide when we need to add (or subtract) an extra "leap second" into our clocks. Leap seconds (if needed) are usually added at the end of the year and are announced well in advance
US Naval Observatory Time Service department
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98.2
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In
this section we consider the consequences of the Earth's revolution around the
Sun once per year. We will come back later to the historical controversy of how
we know that it is the Earth that moves round the Sun, not the Sun round the
Earth
As the Earth moves round the Sun the Sun appears to move against the background of stars
Use Starry Night to show that over the course of one day the Sun moves nearly like the stars, but if you check in detail the Sun moves slightly against the background of stars, as it orbits the earth
As
the Earth moves around the Sun different constellations are visible at night.
The appearance of the sky just after sunset is thus different at different times
of the year.
As the Earth moves around the Sun it traces a path through the sky. This path is called the ecliptic. Note that is has nothing to do with eclipses or ellipses.
Starry night demonstration to show how Sun moves against background of stars. (Zoom out to 180º, turn off sky, advance to noon, step at one day. Then log path with click on "c", then turn on constellations then focus on ecliptic constellations.)
Note that the constellations that the sun passes in front of are the signs of the Zodiac as used by astrologers.
The
direction of revolution around the Sun is the same as the direction of rotation
around the Earth (ignoring the 23½ degree tilt of the Earth's axis for the time
being)
If you look down from above the North Pole the revolution and rotation are both counterclockwise.
Later in the course we will find that many other bodies in the solar system show the same sense of revolution/rotation. The rotation is a clue as to how the Solar System was formed.
Discussion point: What causes the seasons? How can we test the theories?
A number of important effects are caused by the fact that the earth's rotation axis is tilted with respect to its orbit round the Sun
The tilt is 23½ degrees.
The value of this tilt was set at the time of the formation of the Earth
The Earth's axis points in the same direction throughout the year (to Polaris)
There is a slow change in the direction over thousands of years, called precession, but we will not be dealing with that in this course.
The Sun's position on the Sky changes from north to south of the Celestial Equator and back as we go through the year.
Starry night show equatorial view 180 deg, ecliptic and equator
Because of the tilt, the Ecliptic is different from Celestial Equator.
Sun is on the Celestial equator on Sept 21 (Autumnal equinox)
Farthest south in December 21 (Winter solstice)
Back on equator March 21 (Spring equinox)
Farthest North on June 21.(Summer Solstice)
If you are at the Equator the Sun will be to the north of you between March and September, and to the south of you from September to March.
At intermediate latitudes (say 45 degrees north) the sky moves in different daily paths across the sky at different seasons
Some of the consequences are
The
ground gets hotter because the same amount of energy is concentrated into a
smaller area than in the winter
.
Daylight lasts longer in the Summer because the path of the Sun above the horizon is longer in the
summer.
As we move from June to December (Fall in northern hemisphere) the positions of sunrise and sunset move southward. This is easy to monitor. You can only watch ocean sunsets from Waikiki during the winter months!
Northern Hemisphere:
At the North Pole the Sun can be seen for 24 hours in the summer months, and never in the winter months. This is called the Midnight Sun
Above the Arctic Circle (Latitude > 66.5°) the Sun is visible all night for at least part of the summer.
In the northern tropics (Latitude < 23.5°) the Sun transits north of the zenith on at least some summer days. This includes Hawaii.
Generally the farther from the equator you go, the bigger the difference between summer and winter. This is why we get only a small seasonal difference in Hawaii
The seasons are reversed in the southern hemisphere. In Australia it snows in August, and the midnight Sun is seen around the December months.
The elliptical orbit of the Earth causes it to be a bit nearer to the Sun in December than in June. The effect of this on our climate is much less than that caused by the tilt of the Earth's axis.
The object of this section is to understand why the moon looks so different at different times of the month, and why is is seen at different times of the day at different times of the month.
The moon takes about 28 days to orbit the earth. The orbit is in the same sense as the rotation of the earth and revolution of the Sun, namely counterclockwise when viewed from above the Earth's North Pole.. It is tilted by a few degrees from the plane of the Earth's orbit around the Sun.
The Moon does not produce light. The changing appearance of the moon is caused by the changing angles of illumination from the Sun. When the illumination is mainly from the back we see a crescent moon. When it is mainly from the front we see a gibbous moon.
Show appearance of moon from Earth on Starry night. Lock on Moon, orientation ecliptic, FOV about 1 deg. View from center timeskip 2 hours, view from Earth Center. Second window Inner Solar System, center on Earth.
Note that the illumination (or the phase) of the moon does not change much during the course of a night.
At
new moon the Moon passes between the Sun and the Earth. Because of the
tilt of the Moon's orbit with respect to the Earth's orbit around the Sun they
are usually not perfectly aligned. Eclipses occur only rarely.
At true new moon only the back of the moon is illuminated, so we do not see it. Conventionally we often refer to the moon as new when the first crescent is visible a few days after true new moon. The first sighting of the crescent moon after the true "new" moon is of great significance in the Islamic religion. At this time we can sometimes see the dark part of the Moon faintly illuminated by light reflected off the earth.
Next new moon is on February 8th 2004
As we move through the month the moon waxes (its illuminated fraction increases) through first quarter (after one week) to full moon (after two weeks). It is first crescent, then gibbous.
From Full to New again the moon wanes through gibbous as far as third (or last quarter) and crescent to new moon again.
Assuming that you are in an "ordinary" part of the Earth (not near the poles)
At
new moon the Moon passes between the Sun and the Earth. Because of the
tilt of the Moon's orbit with respect to the Earth's orbit around the Sun they
are usually not perfectly aligned. Eclipses occur only rarely.
First quarter seen in evening
Full moon seen all night
Last quarter seen in morning
New moon essentially invisible.
First quarter best for moon viewing.
Moon
keeps same face always pointing towards earth.. This is why we can (in
principle!) recognize the "Man in the Moon"
The only people who have seen the back of the Moon are astronauts.
We will discuss the reasons for the locked rotation later on in the course.
Eclipses occur when the Earth and Moon are well-enough aligned to form shadows on each other
First let's get some idea of the scales
Earth diameter 13,000 km ( 8000 miles)
Moon diameter is about ¼ of Earth's diameter
Moon-earth distance is about 400,000 km. (30x Earth diameter)
Photo of Earth & Moon from Galileo Spacecraft
Lunar
and Solar Eclipse scale diagram
Eclipse
geometry figure. Because the Moon's orbit around the Earth is tilted a few
degrees from the Earth's orbit around the Sun, we don't get eclipses every
month. Usually a couple of solar and a couple of lunar eclipses per year.
Solar Eclipse occurs when the moon comes in front of the Sun
Mpeg movie of annular eclipse (350 kBytes)
The angular diameter of the Moon is almost the same as the angular diameter of the Sun, so you have to be almost perfectly lined up to see a total eclipse. A total eclipse only last a few minutes, max. The effect is spectacular.
Eclipsed
Sun
Solar eclipse occurs only at New Moon.
Partial
eclipse is visible over a large area of the Earth. Total eclipse is visible only
in a small area.
The angular diameter of the Moon (and to a lesser degree, the Sun) vary because the orbits of the Moon and Earth are ellipses. Sometimes the angular diameter of the Moon is larger than the Sun: then you get a total eclipse; if it is smaller than the Sun you get an annular eclipse, which is less spectacular.
Lunar Eclipse occurs when the Earth comes between Sun and Moon. Moon looks darker
A Lunar Eclipse is visible from half the world
It can occur only at Full Moon
The Moon is still faintly visible during a total lunar eclipse, due to scattering of sunlight round the edge of the Earth through its atmosphere. The moon shines with a faint red color.
Web page with dates of eclipses
4:96
Point
above the Earth’s North pole is the North Celestial Pole, very
close to the location of the star Polaris. There is also a South
Celestial Pole, but there is no particular "south star" at
that position. Stars equidistant from the two celestial poles are on the
celestial equator..
How
come we are so sure it is the Earth that rotates, not the sky? You can
do a test with an instrument that is called 
All
stars (except Polaris) move from east to west. They cross an imaginary
line in the sky called the Meridian. The meridian is a line that
starts due south of you, passes directly overhead through the Zenith,
and meets the horizon due north of you. When a star crosses the meridian
we refer to the event as a transit.