In this lesson, we will be using some vocabulary that may be either unfamiliar or may be used differently in an astronomical context than your usual usage. The vocabulary you will encounter in this lesson includes:
- meridian (and transit of the meridian)
All of these terms are used to describe the location or behavior of objects in the sky. For example, you can refer to the "altitude of the Sun". Or, when the Sun passes from one side of the meridian to the other, you can talk about the Sun "transiting the meridian".
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere.
The nadir is the direction pointing directly "below" a particular location, on the imaginary celestial sphere.
The North Point is a point in the horizon in direction of geographic North.
The South Point is a point in the horizon in direction of geographic South.
In astronomy, a vertical circle is a great circle on the celestial sphere that is perpendicular to the horizon. Therefore it passes through the Zenith and the Nadir.
Meridian is a particular Vertical Circle which passes through the North and South Points.
Altitude is the angular distance from horizon to object, meaured along a Vertical Circle.
Azimuth is the angular distance along horizon from N(S) eastwards to Vertical Circle through object for Northern(Southern) hemisphere.
Day and Night:
The most obvious change in the sky is that for about half of the day the sky is brightly lit and, for the other half, it is dark. If the sky is clear, we can tell that the Sun is “up” during the day and “down” at night. Even though this is obvious to most observers, there is one question about day and night that took many centuries to solve:
Is the Earth stationary and is the Sun orbiting it, or vice versa?
We will discuss some of the history of this question (which is a very interesting discussion of the history of astronomy) in Lesson 2, but for now I will directly answer the question.
The Earth rotates around an imaginary line that goes through the North and South Poles, which is called the axis of rotation. So basically when the Earth rotates on its axis and the sun hits the Earth, that part of the Earth is called Day (light). And when the Earth rotates away from the sun, that part of the Earth is called Night (dark).
This has consequences beyond day and night. The Earth's rotation causes every object in the sky to appear to trace out a path on the sky from East to West. So if you picked out a star in the sky, it will appear to make an arc across the sky. This is because, for our purposes, we can consider the star to be fixed in space, and the Earth is rotating underneath it. This happens slowly, so from minute to minute you don't notice the star's motion. However, over the course of a few hours you will be able to tell that the stars have moved a substantial distance on the sky. Since, like the Sun, the stars will (for the most part) appear to rise in the east and set in the west, so the apparent motion of a star will depend on which direction you face. An example is below.
Check out this real observation of the apparent motion of the stars across the sky as seen from Mt. Kilimanjaro.
Credit: Astronomy Picture of the Day
The picture was created by leaving the camera shutter open for more than three hours, showing the path the stars follow in the sky over a small fraction of the night. It also illustrates another important point: the Sun and the stars all appear to rotate around one point, which is the point directly above the North Pole of the Earth for observers in the northern hemisphere (called the North Celestial Pole or NCP), or the point directly above the South Pole of the Earth (the South Celestial Pole or SCP) for observers in the southern hemisphere. There is a star that is positioned very close to the NCP, which is called Polaris, the Pole Star. It is NOT the brightest star in the sky; it isn't even in the top 25. The importance of Polaris is that it roughly marks the location of the NCP, so all stars visible to observers in the northern hemisphere appear to rotate around Polaris.
If you need to know the exact Day/Night situation of the entire Globe, get it from the "Day and Night World Map".
The Path of the Sun:
The sun raises and sets everyday, but the path the sun takes across the sky can differ between seasons.
During the day, we can see the Sun, but the bright daylight sky prevents us from seeing most other objects in the sky (on some days you can see the Moon during the day, and if you know where to look you can also sometimes see Venus). As a thought experiment, think about what you might see if you were able to see the Sun and the stars in the sky during the daytime simultaneously.
- What stars would you see behind the Sun?
- Would you always see the same stars behind the Sun?
The only way to make an experiment on this is using a planetarium software, and setting Daylight off.
Using Stellarium for instance I get this one:
If you could see the Sun and the stars simultaneously, you would see that during the course of one day, the Sun would be inside one constellation (to be more specific, one of the constellations of the Zodiac). To be even more specific, realize that the constellations are made up of stars far in the background, so when we say the Sun is "inside" a constellation, we mean that we are seeing the Sun in projection in front of a specific group of distant stars.
As we discussed at the beginning of the lesson, it is the rotation of the Earth that causes the Sun and the stars to move across the sky, so we should expect that the Sun and the stars should both appear to move at the same rate. Thus, the Sun will be seen inside of the same constellation during the entire day. That is, if the Sun appears to be in the constellation of Gemini at dawn, then it will still be in Gemini at noon and at Sunset.
This is mostly correct, however, there is one effect that we are neglecting to take into consideration. The Earth isn’t just rotating in a fixed spot in space. The Earth is also orbiting around the Sun. In one year, the Earth will make a complete trip around the Sun, so in December the Earth will be on one side of the Sun, and six months later, in June, it will be on the opposite side of the Sun.
So, in December, when the Earth is facing the Sun, the constellation behind the Sun is Sagittarius. Twelve hours later, when the Earth has rotated so that it is night, the Earth is facing directly away from the Sun, towards the constellation of Gemini. In June, the situation is completely reversed because the Earth is on the opposite side of the Sun. The constellation behind the Sun at noon in June is Gemini, and twelve hours later, when the Earth is facing directly away from the Sun, it is pointed towards the constellation of Sagittarius.
This is reasonably easy to visualize when you think of the extreme case of the differences in the position of the Earth six months apart, but what happens on a day to day basis? The way to visualize it is as follows. The stars are so far away from the Earth that, again, for our purposes, we can consider them to be fixed. We know that the rotation of the Earth causes stars to appear to make circles or arcs on the sky that start in the east and move westward. A natural question to ask is, "How long does it take for star A to appear in the same spot in the sky one day later?" That is, let’s say that star A is "transiting your meridian" (this means that if you draw the imaginary line on the sky that connects due North to due South, the star is passing this line at this particular instant in time), how long will it be until star A transits your meridian the next time? You may be tempted to say 24 hours, but the correct answer is 23 hours and 56 minutes! If you do the same exercise for the Sun—that is, if you calculate the time between successive transits of the Sun—it is 24 hours.
The length of time between transits for a star (any star) is called a Sidereal Day, and the length of time between transits for the Sun is called a Solar Day. The difference is caused by the slow drift of the Earth around the Sun. Because the Earth has moved 1/365th of the way around the Sun in a day, it has to rotate more than 360 degrees in order for the Sun to appear in the same part of the sky (e.g., transiting the meridian) as it did yesterday. However, since the stars are so far away, the Earth’s orbit around the Sun doesn’t affect their apparent position in the sky, so the Earth only needs to rotate 360 degrees in order for them to appear in the same part of the sky. Because of this effect, the Sun appears to slowly drift eastward compared to the background stars, and the cumulative effect of this drift is that the Sun will appear to be in Gemini in June and Sagittarius in December.
Note in the figure above that when the Earth rotates 360 degrees it goes from position 1 to position 2, and a distant star will appear to be in the same position as seen from Earth. However, the Earth has to go from position 1 to position 3 for the Sun to appear in the same position.
The Height of the Sun:
We are still not done talking about the apparent changes of the Sun in the sky. You now know that the Sun appears to move from east to west because of the rotation of the Earth, and that if you could see the stars during the daytime it would appear to drift with respect to the stars by a small amount each day because of the orbit of the Earth around the Sun. The next question is:
Does the apparent path of the Sun across the sky change during the year?
Again, let’s consider two extreme cases—December and June. Think about the appearance of the Sun in winter and then in summer. If you live at roughly the same latitude as central Pennsylvania, you should remember that the Sun never gets very high above the horizon in December, but in June it passes almost directly overhead. So the apparent path of the Sun does change from season to season. You can also observe this effect if you re-run the animation on the previous page; in June, the Sun is high above the horizon, in September it is lower, and in December it is very low on the horizon.
The cause of this effect is that the axis of the Earth’s rotation (the imaginary line that passes from the North Pole through the Earth to the South Pole) is tilted with respect to the Sun by an angle of 23.5 degrees. If you go back and look at the animation of the rotating Earth on the day and night page in this lesson, you will see that the line indicating the axis of rotation is not vertical, but is offset by 23.5 degrees from the vertical. As the Earth orbits the Sun, the orientation of the Earth stays fixed, and as a result, in December, the northern hemisphere is tilted away from the Sun during the day, and in June the northern hemisphere is tilted towards the Sun during the day.
There are two consequences of the tilt of the Earth’s rotation axis:
- When the Earth is tilted towards the Sun, the path of the Sun across the sky will be longer than when the Earth is tilted away from the Sun. That is, there are more hours of daylight during summer than there are during winter.
- When the Earth is tilted towards the Sun, the light from the Sun is hitting the Earth more directly than when the Earth is tilted away from the Sun. This means more energy is hitting each square meter of Earth during summer than winter, making summer days hotter than winter days.
Note that the tilt of the Earth is neither towards nor away from the Sun during March and September (Spring and Autumn). Thus, the path of the Sun across the sky and the angle of the Sun’s rays is similar during these two seasons, which is why the length of the day and the daytime temperatures are similar.
There is a nice video that helps illustrate this effect. See Mechanism of The Seasons
Recall that the ecliptic is the path of the Sun across the sky; it can be represented by an imaginary circle in space. If we take the Earth’s equator (another imaginary circle) and project it on the sky, the angle between the ecliptic and the celestial equator would be 23.5 degrees because of the tilt of the Earth. There are four special points on the ecliptic (and note that since the ecliptic is the same thing as the path of the Earth around the Sun, points on the ecliptic are the same things as dates on our calendar):
- Equinoxes - The equinoxes are the two points on the ecliptic where it crosses the celestial equator. On the vernal and autumnal equinox (around March 21 and September 21 respectively) the length of the day and night are roughly equal.
- Solstices - The points on the ecliptic when the Sun is highest above or lowest below the celestial equator are called the solstices. On the winter solstice (around December 21 in the Northern Hemisphere), the night is much longer than the day, and on the summer solstice (around June 21 in the Northern Hemisphere), the day is much longer than the night. See the three figures below for a comparison between the celestial equator (red line) and the ecliptic (green line):
Angle between the Celestial Equator and the Ecliptic on the Summer Solstice
Angle between the Celestial Equator and the Ecliptic on the Winter Solstice
Angle between the Celestial Equator and the Ecliptic on the Autumnal Equinox
So, why do we experience seasons?
This emphasizes one major point that is the most misunderstood fact in astronomy:
The Earth experiences seasons because of the tilt of its axis of rotation. The seasons have nothing to do with the distance of the Earth from the Sun.
There is one observation that should help you remember the cause for the seasons. The seasons are opposite in the northern and southern hemispheres on the Earth. That is, it is summer in Pennsylvania from June through September, but in South Africa, it is wintertime during these same months! This is easy to explain if you understand that the Earth’s tilt causes the seasons; when the northern hemisphere is tilted towards the Sun (summertime), the southern hemisphere is tilted away from the Sun. If the distance between the Earth and the Sun caused the seasons, then it would have to be summer in both the northern and southern hemispheres at the same time, because both would be the same distance from the Sun at the same time. Do you know when the Earth is closest to the Sun? In January!
Higher Sun angle means more luminosity per square meter. Low Sun angle produces fewer rays per square meter. More intensity means more heat and, therefore, higher temperatures.
Note that, due to the fact that our oceans store heat, the actual changes in mean Earth temperature are delayed by several weeks, i.e. the hottest days of summer are usually in late July, over a month from the summer solstice.
Often, when confronted with the understanding that it is the tilt of the Earth's rotation axis that causes the seasons, students who feel strongly that the reason the seasons must be a difference in distance from the Earth to the Sun will point out that the hemisphere tilted towards the Sun is now closer to the Sun. However, the Earth is so far from the Sun that the difference in distance to the Sun between the hemisphere tilted towards the Sun and the one tilted away from the Sun is effectively zero.
Phases of the Moon:
The Moon is tidally locked to the Earth, meaning that one side always faces us (the nearside), whereas the farside is forever hidden from us. In addition, the Moon is illuminated on one side by the Sun, the other side is dark (night).
Which parts are illuminated (daytime) and which parts we see from the Earth are determined by the Moon's orbit around the Earth, what is called the phase of the Moon (click here for the current phase of the Moon).
As the Moon moves counterclockwise around the Earth, the daylight side becomes more and more visible (i.e. we say the Moon is 'waxing'). After full Moon is reached we begin to see more and more of the nighttime side (i.e. we say the Moon is 'waning'). This whole monthly sequence is called the phases of the Moon.
Lunar Phase Simulator.
On rare occasions the Moon comes between the Earth and the Sun (a solar eclipse) or the Moon enters the Earth's shadow (a lunar eclipse).
Eclipses only occur when the line of nodes is aligned with the Sun (2 to 5 times a year).
All solar eclipses occur at new moon with a duration of only 4 to 7 mins. The path of shadow across surface of the Earth determines who gets to see it.
All lunar eclipses occur at full moon and everyone on nightside of Earth is able to observe lunar eclipses. The deep red color during the eclipse comes from light refracted through Earth's atmosphere (i.e. red sunset's)