Earth at Night - From Astronomy Picture of the Day
The telescope wasn't a reality until 1609, but ancient civilizations did some pretty amazing things with just their eyes (and brains). Even if you don't own a telescope or binoculars, you can still make observations about the sky. Some things are obvious to a child; others require the most minute attention to detail and others happen by pure chance. Civilizations in the past had a distinct advantage over most people today ... they could see the night sky much better. Light pollution in the city makes dark sky astronomy impossible. All the incandescent and vapor lights we use today tend to reflect back up toward the sky making all but the brightest stars invisible.
Earth at Night - From
Astronomy Picture of
the Day
So what can you see (even from the city)? Let's go through some of the most obvious things and learn some basic astronomy in the process.
You certainly have noticed that some stars are brighter than others. The brightest ones are usually given proper names. Can you name 5 stars by name? Don't worry ...most people can't ... let alone, point them out in the sky. A keen observer will notice that some stars appear red and others appear blue ... why is that? You will learn why ... later in this project. But just about every ancient civilization would play a game of connect-the-dots with the brightest stars and find patterns and figures within groups of stars. Today, we use 88 separate star pictures or constellations which originated with the Greek/Roman culture. Some of these should sound familiar - Leo, Virgo, Orion, Gemini, etc. Click here or here if you wish to learn more.
If you study the constellations, you are really studying a culture. This is because these early people would incorporate their mythology into these sky pictures. Each one would tell a story, and from that, you can learn about the people who wrote those stories. From a more practical aspect, the constellations help you locate objects in the sky quickly. If you were told that a certain bright planet was in a certain constellation on a given night (and you were familiar with the night sky), you could locate it quickly. But using this method of finding things only works when the target is fairly bright. Also, the constellations often take up a wide section of the sky so this method of finding things only gets you close. For fainter objects, you need a better (and more accurate) way to locate objects, but we won't cover that in this section. Since astronomers only use constellations for the purposes stated above, I will tend to ignore most references to them.
From a very early age you become aware of this change in the sky. When my children were very little (about 2 years old), I would ask them to point where the sun wakes up. To my surprise, they would both grunt and point towards the east window. Watching the sun move across the sky is like watching the hour hand move on a watch (don't ever stare at the sun!). You know it moves, only very slowly. But is the sun really moving? You all know the answers to this ... NO! Even in kindergarten you are taught that you get the day and night because the earth spins on its axis. Yet one of the longest held hypothesis in science stated that the day/night cycle was caused by the motion of the sun around the stationary earth.
Can you offer even one piece of naked eye evidence that could convince someone that we are really standing on a spinning earth (and that is why we get day/night)? Do you feel like you are on a spinning planet? As it turns out, it took a revolution in astronomy to convince people that we really live on a spinning earth. Later we will show you how that story unfolded.
Living in Wisconsin is sometimes a painful reminder that we go through seasonal cycles. This cycle repeats itself year after year. The frozen tundra of Lambeau Field (Go Packers!) gives rise to hot nights at Summerfest. Why does this happen, and what other changes occur in the sky as the seasons dance along?
This answer you also learned in grade school. The earth orbits around the sun once a year. It does so with a tilt (of 23˝ degrees). This means that during our summer, the northern hemisphere is tilted toward the sun .. getting the more direct rays of the sun. In the winter, the sun's rays enter the northern hemisphere at a very shallow angle. When the northern hemisphere is having winter, the southern hemisphere is going through their summer. Sorry if you happen to be reading this from Australia, but I plan to give this project a "northern hemisphere bias".
Why we have seasons.
What other things have you noticed in the sky as the seasons change? How about the length of daylight and the path of the sun across the sky during the course of a day? In Milwaukee, on the first day of summer, the sun rises in the far northeast very early in the morning (33 degrees north of east). It sets very late in the evening in the far northwest (33 degrees north of west). At noon the sun is very high in the sky, and we get a little over 15 hours of daylight. On the first day of winter, the sun rises much later in the day in the southeast (33 degrees south of east) and sets in the southwest. At noon the sun appears very low in the sky ... giving us about 9 hours of daylight.
The sun is high in summer and low in winter.
In addition to these obvious changes throughout the seasons, one can also notice seasonal changes in the nighttime sky. Certain constellations are only visible at a given time of the year. For example, Orion is seen throughout the winter but not in the summer. Sagittarius is just the opposite, only seen in the summer, but not the winter. Maybe you can figure out why this is. If you need a hint, look at the diagram above showing the earth going around the sun throughout the year.
There are some things you should already know about the moon. It goes around the earth, it only reflects sunlight (does not produce light on its own), and is NOT made of green cheese. You are also aware that it goes through a cycle of phases. There are 4 major phases - new, 1st quarter, full and 3rd quarter. The animation below shows all 4 phases and why we see them.
This animation shows the 4 major moon phases (as seen from the northern hemisphere).
We see the phases as a result of the way the moon presents itself to us (in relation to the sun). At any one time, the moon (like any spherical body) is one half illuminated by the sun, and the other half is in darkness. It should be clear from the animation above that as we watch the moon from the earth (blue sphere), we sometimes see none of that illumination (new moon) ... sometime all of the illumination (full moon) and sometimes only half of the illuminated side (1st and 3rd quarters). In this animation, you are looking down on our solar system from above. The red dot represents the north pole of the earth. Imagine you are standing there, facing the moon. Study this animation carefully, and make sure it is clear in your mind why we see the moon going through these major phases. Be able to instantly identify a new or full moon alignment (between the sun, earth and moon).
You might be wondering why the moon is called 1st or 3rd quarter when it is clearly only half illuminated from our perspective. The name is a reference to time and not shape.
On rare instances you get the privilege of witnessing an eclipse. Nature provides us with two varieties - lunar and solar.
Lunar Eclipses - This occurs when the moon moves into the earth's shadow.
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A lunar eclipse is only possible on the night of a full moon! |
Photo taken by me on 1-20-2000 |
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A solar eclipse is only possible on the day of a new moon. |
Photo taken by me on 8-11-1999 |
Ancient civilizations were able to recognize 5 planets (the earth was not considered a planet) - Mercury, Venus, Mars, Jupiter, and Saturn. It wasn't until well after the invention of the telescope that Uranus, Neptune, and Pluto ( sorry Pluto ... but you are officially off the list) were found because they are too dim to see with the naked eye. There were a number of reasons the planets were considered different from the stars (even though the naked eye makes both out as single points of light). If you were to watch and study the constellations, you would notice that their position changes on a daily and seasonal basis ... but they always stay the same relative to each other. That is, they move together in the sky ... Aries is always next to Pisces which is next to Aquarius, etc. The planets, however, move from one constellation to the other. This is why an astrologer would ask for your date of birth ... so they could determine which constellation each planet was in when you were born. These "wandering stars" were usually considered gods by early cultures. The animation below shows how Mars appears to move relative to the "fixed" stars. Note: Mars does appear red to the naked eye (can you see why it was considered the god of war) ... but so do several stars such as Antares, and Betelgeuse. The motion of a planet against the stars may take several days or even months to notice (depending on the planet and where it is at the time of observation).
The motion of Mars against the stars
In a hurry? There is an easier way to identify planets. Everyone has heard the nursery rhyme, Twinkle, Twinkle little star! The stars twinkle because of an atmospheric disturbance. Even though stars are physically much bigger than planets, they are so much further away that they appear as mere points of light to the eye. This also makes their rays easy to redirect in the ever changing layers of the atmosphere ... making them appear to twinkle. Planets, however, do not twinkle because they actually are close enough to appear as a small disk (even though your eye can not detect this). This tends to wash out the altering effects of the atmosphere and they shine with a steady light. This is a quick way of picking planets from stars.
There is one other way to make a distinction between planets and stars. The brightness of planets changes with time, but some planets (Venus, Mars and Jupiter) will appear much brighter at times than any visible star. So if you happen to see a point of light in the sky that far outshines any of the surrounding bright stars, you can bet it is a planet.
Comet Hale Bopp (1997) taken from my backyard
Comets have traditionally been taken as a prophesy of impending doom or disaster. They certainly could be if they were to hit the earth (which they have done in the past). Some comets make predicable returns to our skies (like Halley's Comet) and others are new discoveries (several are found every year). But what are these puzzling objects which dazzle us with their mysterious tail? They are just dirty snowballs ... only much bigger. A typical comet is a block of ice with solids mixed in which happens to be about the size of a mountain. You can imagine the results of an impact with the earth - a possible extinction event.
Most comets enter the inner solar system from great distances (where it is much colder). How they happen to get here is a matter we will take up later in this project. When they do visit our area of the solar system (where it is much warmer), the ice is no longer stable. Some of it vaporizes ... first forming a fuzzy haze around the solid ice ... and then a long tail. If they leave our warm section of the solar system, the temperature drops ... the ice becomes stable ... and the tail disappears, ... and we dodged another bullet.
You have all witnessed a "shooting star" in the sky (I hope). These brief streaks of light really have a better name - meteors. What you are seeing is a solid piece of matter burning up in the atmosphere. Most students are surprised when they discover that the original size of a typical meteor ranges from the size of a pea to a grain of sand ... and they are burning up in the atmosphere 80 miles above their heads. If the rock is bigger, it has a good chance of making it to the ground. In 1992 in Peekskill NY, a 12.4 kg fragment smashed the back end of a parked car (nobody was hurt). During its descent, it produced a magnificent "fireball" which was seen from several states. Imagine what would happen if a rock the size of a house (or bigger) were to strike the earth!
Meteors can occur anytime, but they are often witnessed in great numbers in an event known as a "meteor shower". Probably the best meteor shower occurs every summer around August 10-12 and is known as the Perseids, but there are about a dozen predictable meteor showers throughout the year worth seeing. How and why we get these annual events may seem like a mystery, but there is really a simple answer. Every once in a while, a comet enters the inner solar system where it is much warmer. As the ice vaporized, small pieces of solid matter "flakes off" the comet and leaves a trail of debris in its path. If the comet's path crosses the orbital path of the earth, the earth strikes that debris every year (consider it lucky that the earth wasn't at that spot the same time as the comet).
An Aurora (Northern Lights) taken from my backyard
An aurora is an atmospheric event. One could argue that this event, therefore, belongs in a meteorology class and not an astronomy class. However, I would suggest the opposite. If you understand that this event is caused by activity on the sun, we must include it on the list. Our sun goes through cycles which results in high numbers of sunspots and other solar events at times. When solar activity is at its highest, sunspots are even visible to the naked eye (when clouds provide the appropriate conditions). Warning: Never look at the sun without proper safety equipment (like #14 welders glass)! An aurora is just a gentle reminder that activity on the sun can produce radical changes to the earth. We will cover these effects in detail later in this project.
Only the best (or luckiest) observers notice changes in the stars. It would take thousands of generations to notice changes in position (where the constellations actually look different), but some stars change their brightness on much shorter time scales.
Some stars pulsate on time periods of over a year to just a few hours. These pulsations result from physical changes in a star's diameter. This changes the radiating surface as well as the temperature ... both of which effects a star's light output. One such star, Mira, was observed by David Fabricius in 1596 (about 13 years before the use of telescopes). Mira's brightness changes by a factor of 100 in the course of a year.
Another star, Algol in Perseus becomes 3 times dimmer for a few hours, but for a different reason. The name Algol means "demon star" because ancient astronomers thought it was cursed (because its brightness changes). Algol is actually two stars that orbit around each other in our line of sight. The brightness change results when the fainter star eclipses the brighter one.
Some stars flare up on a regular basis and others on their own timetable. When they do, their brightness increases significantly. The name "nova'' is Latin for "new" and refers to the fact that a new star appears where none was seen before. One way to produce a nova is to have two stars orbit each other in such a way that material is drawn off one star onto the surface of the other, resulting in an explosion on a regular basis. Other novae occur when an unstable star "blows off" layers of its atmosphere.
Probably the most dramatic change in brightness occurs when a star blows up completely. There have only been a handful of these events recorded in human history. Known as supernovae, these events are rare and also (as you will find out) very important in shaping our universe. The last naked eye supernova occurred in 1987, but was not visible from Wisconsin ... :(
As you can see, there is a great deal of astronomy that can be observed without the use of instruments. The list above is a preview of things we will discuss in greater detail at various points in this project.
ŠJim Mihal 2004, 2014- all rights reserved
