It is often thought, particularly by beginners, that amateur astronomy requires the purchase of a telescope of some kind. Certainly it is true that owning a good quality telescope does make it possible to observe many more objects in the sky than can be seen without one. However, the fact is that a lot of interesting observing can be done with just the naked eye as well. Some amateurs, even experienced ones, sometimes believe that naked eye observing is only suitable for novices. However, that is not true either. In fact, there are many interesting and worthwhile naked eye projects for even the most advanced observers.
In this document I will talk about some of the options for naked eye observing. Some of these projects are good for beginners and some are suitable for only the most dedicated observer. Some of these projects require very dark skies and some can be done even in the city. If you have never considered the options for naked eye observing before, you might be surprised at the opportunities it presents!
Sky conditions are very important to amateur astronomy. One useful naked eye project, regardless of the sort of observing you usually do, is to just monitor the conditions of the sky at your location. Keep a weather log and keep track of which days are cloudy and which are clear. Statistics on temperature and humidity might also be interesting. Note the naked eye limiting magnitude. Is the Milky Way visible? If so, how well? Look both at the zenith and toward the horizon. The sky quality will degrade near the horizon, but by how much? As a naked eye observer you can get some idea of the quality of the "seeing" by observing star twinkling.
Light pollution causes major problems with astronomy. How bad is the light pollution at your location? Which sections of your sky are affected? Light pollution tends to become less of a problem late at night because many lights are shut off at those hours. As a naked eye observer, you might explore how the light pollution at your location varies with time of night, season of the year (does it correlate with the amount of snow cover?), or the weather.
The Moon is a favorite object for observers of all kinds. Here are some ways you can enjoy the Moon as a naked eye observer.
Some observers spend a great deal of effort locating the Moon as soon as possible after it is new (or as late as possible before it becomes new). If you have a good horizon view to either the east or west, this might be a good project for you.
Fairly regularly the Moon enters the shadow of the Earth and is eclipsed. These events are often best viewed with the naked eye. Because of the way light refracts in the Earth's atmosphere, the eclipsed Moon is almost never totally dark. Observing just how dark the moon gets during an eclipse is one of the most interesting aspects of the event.
Do you know where Jupiter is right now? How about Mercury? A faithful naked eye observer should be familiar with the locations of all the major planets: Jupiter, Saturn, Mars, and Venus. How soon can you locate these planets after their conjunctions with the sun?
Searching for Mercury is a valuable naked eye project. The planet has several elongations from the sun each year. Can you catch the innermost planet at each of those elongations?
Meteors will always have a special place in the heart of a naked eye observer. Although meteors can be observed with a telescope, a radio, or even with a video camera, the naked eye is one of the best instruments for the task.
Meteor observation is a place where you can even make a meaningful contribution to the study of astronomy. Careful observations of the number of meteors you see and their apparent paths can contribute to our knowledge of important meteor showers. See the web pages of the International Meteor Organization and the North American Meteor Network for more information. The NAMN, in particular, is dedicated to coordinating the observations of ordinary amateurs.
Occasionally comets reach naked eye brightness and can be tracked without optical aid. When such a comet appears be sure to watch its tail. Often comet tails exhibit interesting behavior.
There are also a few asteroids that reach naked eye brightness, but a dark sky and accurate finder charts would be necessary to observe them. The planet Uranus is also in this category.
Some stars vary in brightness. With a good finder chart, especially one that shows appropriate comparison stars, you can follow the brightness variations of the brighter variables. The American Association of Variable Star Observers might be able to provide you with the necessary information.
Most of the naked eye variables are not especially interesting from a scientific point of view, but they are potentially interesting to follow anyway. Algol and Mira are two stars that come to mind right away.
One activity where a naked eye observer might be able to contribute to science is in the search for nova. To be effective at this, you will need an encyclopedic knowledge of the sky. You will need to know all the naked eye stars down to at least 4th magnitude or fainter. By scanning the sky each night looking for a "new" star you might well be the first to pick up a Nova on the rise.
People do not normally associate observing distant galaxies with naked eye observing. However, with a dark sky there are a number of deep sky objects that you can observe without optical aid. The galaxy M31 is the classic example. There are also several clusters and even some nebula (such as the Orion Nebula) that are visible to the naked eye.
The naked eye is also an excellent instrument for probing the dark lanes of our own galaxy. If the Milky Way is bright, you can generally seen large patches of darkness amid the sea of stars.
There are several other phenomena that you can look for without optical aid.
There are many "artificial" satellites orbiting the Earth. They are best observed shortly after sunset or before sunrise when the light from the sun is illuminating the space immediately overhead.
If you have a dark northern view you can search for aurorial displays. Some displays are very spectacular and fill the entire sky. But many displays are not that extensive and are easily missed. If you specifically spend time looking for aurora you might be surprised at how often you can see it. Note that auroral displays are more common and more spectacular at higher latitudes. If you live in the far South, aurora will be rare.
This is a faint, conical "beam" of light that appears in the west right after sunset or in the east right before sunrise. It follows the ecliptic and is due to sunlight scattering of dust particles orbiting the sun.
This is a faint patch of light, similar to the Zodiac light, except that it occurs exactly opposite the sun in the sky. It is best seen when the that point is as high as possible (winter months for Northern Hemisphere observers). It is caused by scattering of sunlight from dust particles orbiting the sun that are directly "behind" the Earth.
I understand that under certain conditions, immediately after sunset while the sun is still illuminating much of the sky, it is sometimes possible to see the shadow of the Earth pointing outwards to the east (opposite the sun, of course) and into space. This only occurs when the atmospheric conditions allow it. What you are really seeing is the shadow being cast on the haze and impurities in the upper atmosphere. It is not easy to observe.
This is an exceedingly rare phenomenon. Under certain conditions, just as the sun sets (or just as it rises) the sky will appear to light up green for a moment. The Green Flash is best observed if you have a horizon view of the sunset (or sunrise). It tends to be observed most often at sea.