University Lowbrow Astronomers

University Lowbrow Astronomers
Naked Eye Observer’s Guide

by Dave Snyder
Written: February, 2000
Revised: September, 2005

This page has a variety of suggestions for beginning amateur astronomers who wish to observe the sky with only the unaided eye. It is a common misconception that you need to have a telescope to do astronomy; this simply is not true. A wide variety of objects can be seen with the naked eye: from planets and stars, to nebulae and galaxies. While you can usually see more detail if you use a telescope (or binoculars) and you can usually see more objects of a given type if you use a telescope (or binoculars); the same types of objects that can be seen with a telescope can be seen without a telescope.

If you’ve decided you want to observe the night sky without a telescope, you will need some idea of what to look for. Even if you have a telescope or have made a firm decision to purchase a telescope, it is a good idea to look at the sky without optical aid. The time you spend will give you ideas on where to look and what to look for in a telescope. It will also enhance your observing skills.

Magnitudes

Some things are brighter than others: Astronomers use an extension of a system invented in the second century B. C. to describe brightness of objects. Most objects are assigned positive numbers: The larger the number the dimmer the object (magnitude 3 is dimmer than magnitude 2). Very bright objects are assigned negative numbers; this is true of the sun, the moon, bright planets and the brightest stars. Again the larger the number, the dimmer the object (magnitude -2 is dimmer than magnitude -3).

Here are a few examples (note these are typical values, values may be larger or smaller under some conditions):

  Magnitude -27

  The Sun

  Magnitude -12

  The Full Moon

  Magnitude -5

  Venus (the brightest planet) at its brightest

  Magnitude -1

  Sirius and Canopus (the two brightest stars)

  Magnitude 6

  The dimmest objects visible with the naked eye  

  Magnitude 10

  The dimmest objects visible with binoculars

  Magnitude 25

  The dimmest objects on a photographic plate
  produced with a large telescope

As mentioned above, the dimmest object visible with the naked eye is typically magnitude 6.5. However this magnitude (called the magnitude limit) can vary. Many factors affect the magnitude limit:

For information about light pollution and what you can do about it.

Observing Sites

The first step in astronomical observing is selecting an observing location. If you planning to observe bright objects, such as planets, it may be best to pick a site that is convenient, such as a place close to home. If you are planning to observe dim objects you should pick an observing site that maximizes your limiting magnitude. It often helps to observe near the time of the new moon, so the moon doesn’t drown out the light from dim objects. In many cases it is helpful if there is a good horizon; such a location will have few obstructions such as hills, mountains, trees or buildings. It is also a very good idea to abstain from alcohol and nicotine before and during an observing session.

The limiting magnitude at suburban locations is typically 3.5, and the limiting magnitude at dark sites is typically 6.5. Experienced observers at very dark sites have reported limiting magnitudes as high as 8.5.

Constellations

If you want to find your way around the sky, you need to learn the constellations. Constellations are groupings of stars created by astronomers; many of the modern constellations were borrowed from constellations invented by ancient peoples. The existing constellations are not always the best groupings possible; there are many groupings of stars that seem like they should be constellations, but are not. Such groupings are called asterisms.

Which constellations are visible vary depending on your latitude, the time of day and the time of year. In general, at a given location, some constellations will always be visible (they are called circumpolar), some are visible part of the time, and some are never visible.

One way to learn constellations is to first learn a few landmarks. The best landmarks vary depending on time of night and the season, in the Northern Hemisphere the following are good choices:

  Season

  Season   Landmark

  Winter Evening

  Autumn Morning   Orion

  Spring Evening

  Winter Morning   Big Dipper

  Summer Evening  

  Spring Morning   Summer Triangle

  Autumn Evening

  Summer Morning    Great Square of Pegasus 

Each landmark can be used to locate other stars and/or constellations. (Note: Orion is a constellation; however the Big Dipper, the Summer Triangle and the Great Square of Pegasus are asterisms. The indicated times are not the only times these landmarks are visible; for example the Big Dipper is circumpolar from most Northern Hemisphere locations, it is visible at all times. However spring evenings and winter mornings are the best time to use the Big Dipper as a landmark.) As you learn the constellations, watch out for asterisms that look similar to constellations; if you are not careful you will find the sky filled with “big dippers” or “great squares.” Try not to go by shape alone.

For more information on using landmarks to locate constellations, go to Introduction to the Constellations.

For on-line information on the constellations, go to Dibon Smith: The Constellations.

Finding Your Way Around

To find your way around the night sky, you probably will find a star chart helpful. You have several choices:

Star charts may be somewhat confusing at first; they never look exactly like the night sky. However, this confusion will disappear once you get used to them.

For more information:

The Planets

Venus, Mars, Jupiter and Saturn are easy to observe with the unaided eye. Each planet is visible within a fixed schedule. For a given evening, each planet may be visible the entire evening, just part of the evening or not be visible at all. This schedule doesn’t change by much over the course of an evening or two, but will change over a period of weeks or months. Over the course of the year, each of these planets will be visible at least part of the time. It is not possible to see any detail on these planets without a telescope.

Whenever Venus is visible, it is much brighter than anything in the sky with the exception of the sun and moon. Mercury, Mars, Jupiter and Saturn are brighter than anything other than Venus, each other, the sun or moon, Sirius and Canopus.

There are several things you can notice if you observe planets over an extended period of time. Some planets display noticeable changes in brightness over the course of time. This is particularly true of Mars: sometimes it is almost as bright as Venus but can get as dim as magnitude +2. When Mars is bright, it is bright red; when it is dim, it is a pale pink (at such times it may be difficult to see any color at all). Mars goes from bright to dim back to bright every two years. Changes can be observed in the other planets, but they are less obvious. All planets move relative to the background stars, but Mars, Jupiter and Saturn will occasionally appear to go into “retrograde” motion (they normally move in a preferred direction, but occasionally they reverse direction, move in that direction for a time, reverse direction again and move in their preferred direction).

While Mars, Jupiter and Saturn all have satellites (moons), only the four brightest satellites of Jupiter are bright enough to be seen with the unaided eye. However even these satellites are almost impossible to observe in practice (without binoculars or a telescope) since they almost always get lost in Jupiter’s glare.

To observe Mercury, you must plan ahead. The best opportunity to observe Mercury is when it is close to maximum elongation (that is, the planet appears at its highest point in the sky). It also helps to observe from a location without obstructions (such as buildings, trees, hills or mountains) in the direction of the sun. Given the right conditions Mercury will be very bright and can be seen with the naked eye.

When Mercury is visible, you have only a short period of time to observe it. Sometimes Mercury is visible in the evening. In this case you can start observing Mercury shortly after sunset but you will have less than an hour until the planet sets below the horizon. At other times Mercury is visible in the morning. You will be able to see Mercury at most an hour or so before sun rise and can observe it until the light from the rising sun drowns it out. Note: the amount of time that Mercury is visible depends on the position of Mercury in its orbit, the elongation and your horizon; Mercury may be visible for only a few minutes or may not be visible at all if your horizon is poor.

Both Venus and Mercury occasionally pass between the Earth and the Sun, such an event is called a transit (for more information on transits, see the next section “The Sun”).

Uranus is bright enough to be seen with the unaided eye, but appears as a dot of light that can be very difficult to distinguish from nearby stars.

Neptune and Pluto are too dim to be seen with the naked eye.

On occasion a planet passes in front of a star or the moon passes in front of a planet, such an event is called an occultation. Occultations can be predicted in advance and are easy to observe if you plan ahead. Note, the timing of an occultation depends on your exact location. A given occultation is visible from some locations on the earth and not from others.

It is often possible to see planets during the daytime. The trick is to know exactly when and where to look. Venus is the easiest to locate, other planets are harder.

For more information:

The Sun

Several solar phenomena are visible with the naked eye. WARNING: it can be dangerous to observe the phenomena described in this section. If you do not have proper eye protection you may suffer permanent eye damage. It is safe to observe the sun at sunrise and sunset provided you do so without magnification. However observing the sun with binoculars or telescopes is especially dangerous, this can damage your eyes and your binoculars/telescope. This is true even at sunrise/sunset. Sunglasses and smoked glass are not adequate protection. These phenomena include sunspots, solar eclipses and Venus transits (a transit is where a planet passes between the sun and the earth, the planet can be seen as a black dot on the sun’s disk).

Observing sunspots is relatively easy. During solar maximum, which occurs once every 11 years, sunspots are usually quite numerous. At such times, if you have proper eye protection, it is possible to see sunspots (no magnification is needed). (Eye protection is unnecessary at sunrise/sunset provided you use no magnification). At solar minimum (which also occurs every 11 years), there are generally very few sunspots visible.

Observing a solar eclipse requires planning ahead. Eclipses occur infrequently: in a given year there may be as few as two solar eclipses or as many as five. A given eclipse is visible only in some parts of the world. Some solar eclipses are partial (the moon covers only part of the sun’s disk) and others are total (the moon covers the sun’s entire disk). If a solar eclipse is total, totality can only be observed for locations on the earth within a narrow band which is at most 170 miles or 270 kilometers wide but thousands of miles/kilometers long. Outside this zone a total eclipse appears as a partial eclipse; the non-total parts of a total eclipse and partial eclipses are often visible from over half the surface of the earth (assuming there are no clouds). Totality can last as long as seven and one half minutes, but is usually shorter (and the duration varies depending on your exact location). Observing solar eclipses is more dangerous than observing other solar phenomena. It is safe to look at the sun during totality, but you must either look away or put on eye protection the moment totality ends. You normally have a strong reflex reaction which prevents you from looking at the sun for any period of time, however this reflex does not operate during an eclipse, this is the reason people suffer blindness during an eclipse.

Transits of Venus occur either once or twice every 121 and a half years. If there are two transits in this period of time, those two transits are eight years apart. That is the case during the 21st century: the last one was on June 8, 2004 and the next one willl be on June 6, 2012. If you have proper eye protection, you can see the disk of Venus without magnification. A Venus transit can last as long as 7.9 hours. Such a transit will be partially visible from well over half of the planet earth, but the complete transit is visible from only about one third of the globe. However most Venus transits do not last that long and are visible over a smaller area (the duration of the transit varies depending on the location you observe from). For more information see http://www.transitofvenus.org/.

Transits of Mercury are more common than transits of Venus: Mercury transits occur on average every eight years (though the pattern is not regular). However Mercury’s disk during a transit is too small to be seen with the unaided eye (you must use a telescope with a proper solar filter).

A number of phenomena are created by light from the sun interacting with the atmosphere. These phenomena include rainbows, haloes, solar pillars, crepuscular rays, coronas, sundogs, glories and the green flash. Observing the green flash is safe as long as you look at sun only while it is red; observing rainbows is always safe; observing halos, solar pillars, crepuscular rays, coronas and sun glories is safe as long as you make a reasonable effort to avoid looking directly at the sun. Never point binoculars or a telescope directly at the sun even during sunrise or sunset.

For more information:

The Moon

The first thing you will notice about the moon is that it changes phase over the course of its 29 1/2-day cycle. The phases are listed below (this shows the number of days into the cycle and the period of time during which the moon is visible. These times are approximate and do not take into account daylight savings time or variations due to longitude).

1 day   

  New Moon  not visible

4 days  

  Waxing Crescent  8 AM to 10 PM

7 days  

  First Quarter  11 AM to 1 AM  

10 days  

  Waxing Gibbous  2 PM to 4 AM

14 days  

  Full Moon  5 PM to 7 AM

18 days  

  Waning Gibbous  8 PM to 10 AM

22 days  

  Last Quarter  11 PM to 1 PM

  26 days  

  Waning Crescent    2 AM to 4 PM

You will note the moon is visible during daytime hours a significant fraction of the time.

The crescent moon points to the position of the sun (except during the new moon, when the moon is not visible, and during the full moon, when there is no crescent). The moon often appears to point slightly above or below the sun, but this is an optical illusion - it really does point to the sun.

You may have heard that the same part of the moon always points to the earth and half the moon can never be seen. This is only partly true; due to a phenomenon called libration you can see 59 of the moon provided you observe over a period of time. The moon wobbles very slowly when it orbits the earth, and this wobble is noticeable if you compare the edge of the moon day after day for a month or so.

With even a causal look at the moon, you can see maria (which are lowlands and dark colored), terrae (which are highlands and light colored) and the terminator (the line between the dark part and the lit part of the moon). Features that are harder to see include craters and mountain ranges. Despite what you might think, the full moon is not the best time to look for lunar features; maria and terrae are easiest to see during the full moon, but craters and other features are easier to see when the sun is low in the lunar sky as seen by that feature. In other words it is best to observe the moon at several different phases (particularly first quarter and last quarter). Lunar craters are too small to be seen without optical aid, however there is one mountain range that can be seen with the naked eye. If you look just after first quarter, the mountain range Apennines can be seen as irregular section of the terminator (which is normally a smooth arc).

Lunar eclipses occur when the Earth passes between the Sun and the Moon. In a given year there may be as few as two lunar eclipses or as many as five; some of these are partial eclipses and some are total. An eclipse is divided into three stages, a “penumbral” stage when the moon is only slightly darkened, an “umbral” stage when the moon is very dark and another “penumbral” stage when the moon is again only slightly darkened. In some partial eclipses there is no umbral stage, and the entire eclipse is penumbral. In these situations, the eclipse is easy to miss. A Lunar Eclipse can last as long as one and three quarters hour, but are usually shorter.

On occasion the moon passes in front of a planet or a star, such an event is called an occultation. Occultations can be predicted in advance and are easy to observe if you plan ahead. Note, the timing of an occultation depends on your exact location. A given occultation is visible from some locations on the earth and not from others.

For more information:

Asteroids

Asteroids are small bodies, most of which are in orbits between Mars and Jupiter. All are faint.

The best time to observe an asteroid is when it is at opposition (that is the time it is closest to the earth). If it is visible at all, an asteroid will be visible only as a dot of light that is difficult to distinguish from nearby stars. Of the hundreds of known asteroids, Vesta is the easiest to observe and will sometimes be visible to the naked eye. Experienced observers at very dark sites may be able to see asteroids in addition to Vesta.

For more information see the IAU: Minor Planet Center. This site describes many asteroids that are not naked eye visible, pay attention to the magnitudes listed on this site.

Comets

Some comets are short-period comets, that is they return after a set period, which can be as short as three years or as long as 200 years (any comet with a period longer than 200 years is not considered to be short-period). The best known of these comets is Comet Haley, which returns every 76 years. Many short-period comets are known, but not all comets are short-period. New comets are continually being discovered. Most newly discovered comets are not short-period; they will not return for hundreds or thousands of years.

Comets generally spend most of their time far away from the sun. They are close to the sun for a relatively short time, but this is the only time they are easy to observe. New comets are often detected far from the sun (when they are quite dim). Usually such comets are getting closer to the sun, if so they can be expected to get brighter over time. Astronomers often make predictions about how much brighter these comets will get, but such predictions are frequently wrong (Comet Kohoutek in 1973 was much dimmer than predicted, Comet West in 1975 was much brighter than predicted).

At any point in time, over a dozen comets can be seen through telescopes; however most are not naked eye visible. Comets that have bright easily visible tails occur roughly once every 10 years, comets that are naked eye visible with small fuzzy tails are somewhat more common. Comets have at least two tails, a dust tail and an ion tail; the dust tail is the easiest to see, the ion tail is visible only in some very bright comets. When it is visible, the ion tail is oriented at a slight angle to the dust tail and is blue in color.

For a list of comets that are currently visible, see IAU: Astronomical Headlines. Note, most of the comets listed on this page are too dim to be seen without optical aid; pay attention to the magnitudes of the listed comets.

Interplanetary Dust

Dust within the solar system can reflect light from the sun, and this reflected light can be seen from a dark site. Zodiacal light is a soft glow that is usually triangular in shape. There are two areas of zodiacal light, one in the east and one in the west; they are easier to see in winter. The zodiacal band is a band of light that joins the two areas of zodiacal light, but it is very difficult to see. An oval shaped section of the zodiacal band, the Gegenshein often is bright enough to be seen even when the zodiacal band itself cannot be seen.

Aurorae

Aurorae are bright lights visible at night caused by electrons entering the earth’s upper atmosphere. Aurorae in the Northern Hemisphere are called Aurorae Borealis or Northern Lights; aurorae in the Southern Hemisphere are called Aurorae Australis or Southern Lights.

Aurorae are more frequent within an oval about 2,500 kilometers wide centered on the North Magnetic Pole (located in Northern Canada) and within an oval about 2,500 kilometers wide centered on the South Magnetic Pole (located in the ocean near Antarctica). This means the Aurorae Borealis is most commonly seen from Alaska, Canada, Greenland and Iceland. It can be seen, but less frequently from the northern continental United States, Scandinavia, Ireland and the United Kingdom. On rare occasions, aurora have been seen even further from the north magnetic pole. The Aurorae Australis is rarely seen since there are no populated areas near the South Magnetic Pole.

Aurora are more common at solar maximum (which occurs every 11 years). Aurora can be relatively common up to 3 years after the solar maximum. Scientists have been somewhat successful (but not perfect) at predicting when aurorae will occur.

Aurorae can appear in several different shapes, but the most common shapes have been described as sheets, curtains and streamers. They can appear in different colors (green is the most common, however red, violet and blue have been observed). Under unusual conditions, aurora can appear to pulse on and off over a large section of the sky (the author has observed this only once). Aurorae observing does not require dark skies, but colors and pulsation are subtle phenomena which are easier to observe if you are dark adapted and observing from a dark site.

For more information, see

Meteors

On any clear night you can expect to see a number of meteors (meteors are sometimes incorrectly called “shooting stars”). The number you will see is affected by several factors.

The main meteor showers are shown below (the dates given below may vary slightly from year to year).

If you plan to observe meteors, your best bet is to wait for a major shower, such as the Perseids. You can attempt to observe any night of the year, but you may observe for a long time and not see anything. It is best to bring a chair, anything else you need to be comfortable (perhaps warm clothing), sit back, relax, and watch. Typical non-shower observation rates at midnight at a dark sight during March are zero to 3 per hour; typical observation rates during September (midnight at a dark sight) are 8 to 10 per hour.

With a simple radio (in the United States use an FM radio) it is possible to detect the presence of meteors. This can be done during the daytime or nighttime, when it is clear or when it is cloudy. You may want to consider trying this if it is overcast during a meteor shower.

For more information

Artificial Satellites

There are thousands of man-made satellites in orbit around the earth. Some are too dim to be seen with the unaided eye. However others can easily be seen if you know when and where to look. In a given evening, you can expect to see several satellites; they appear as dots of light that slowly move across the sky.

For more information see “What Satellite is That?” by Christopher Sarnecki, June, 1999.

Galaxies

The universe is composed of billions of galaxies. If you go outside at night, almost all the objects you can see with the unaided eye are within our own Milky Way galaxy. There are four exceptions:

Our Galaxy, the Milky Way, can be seen from any reasonably dark site as a band of light that extends across the sky.

Stars

In general all stars vary slightly in brightness over time. Most of these changes are insignificant, however some stars vary noticeably in brightness. The majority of these so-called variable stars change in brightness over a period somewhere between a few hours and a year. A few such stars can be seen with the naked eye; if one is careful, persistent and patient, it is possible to track the variations of these stars. In some cases the star is visible during part of its range and not visible during the rest of its range.

Two types of variable stars, novae and supernovae, change brightness dramatically and unpredictably. Novae will increase in brightness from 7 to 16 magnitudes over a day or two and then decrease in brightness slowly over the next several months. Novae bright enough to be seen with the naked eye occur once every few years. Supernovae increase in brightness over 20 magnitudes in the span of a few days and then decline slowly over the next few years. Bright supernovae are rare. Over the past 400 years, there have been only three naked eye supernovae, one in the year 1604, one in 1885 and one in 1987. Both novae and supernovae are unpredictable. It is possible to locate a previously undiscovered novae or supernovae with persistence and luck.

The color of stars is easily observed. Color is directly related to the star’s temperature. The coldest stars are red; the hottest stars are blue-white. Stars with intermediate temperatures can be orange, yellow or white. It is easy to tell the color of bright stars, but difficult to tell the color of the dimmer stars (dim stars often appear white even though they may be red or blue or some other color).

Many of the points of light that appear to be stars are in fact double stars or even triple or quadruple stars. In a few cases, it is possible to “split” these stars (that is observe the different components) with the naked eye.

In some cases, stars can be found in large groups called “clusters.” There are two types of clusters, globular and open. There are a few globular clusters and a few open clusters visible with the naked eye (they include Omega Centauri, M13 and the Pleiades).

In addition, stars can be grouped into either associations or streams these are stars that are loosely bound but are gradually moving away from each other. Many associations and streams are visible, however it is not always easy to tell which stars are included and which are not. Two of the most obvious associations involve some of the brightest stars in Ursa Major and Orion.

For more information:

Nebulae

Nebulae are areas of gas within the galaxy. Some nebulae (emission, reflection and planetary nebulae) either glow or reflect light from nearby stars; many such nebulae can be seen from the earth. Other nebulae (dark nebulae) do not glow, but can be detected because stars cannot be seen through them (these nebulae are opaque and starlight cannot pass through them). A few nebulae are visible to the unaided eye, they include:

For more information see The National Deep Sky Observers Society (Observing the Deep Sky with the Naked Eye).

Night Myopia

Many people suffer from near-sightedness or myopia, and they correct for it with eyeglasses or contact lenses. It has been known for some time that myopia often gets worse when observing at night. This is called night myopia. It’s not a serious condition, and most people don’t realize they have it. It’s temporary and goes away during the daytime. However if you suffer from this condition, your night visible isn’t quite as good as it could be and you may not be able to see some of the objects described earlier in this article.

If you use a telescope or binoculars, the focus knob will allow you to compensate for night myopia. However this doesn’t work for naked eye observing.

The first step is to determine if this situation applies to you (each person’s eyes are different). There is a company that sells a “night myopia flipper” set. These are lens that you can place over your eyeglasses (or contact lens) and determine if additional correction is necessary to deal with night myopia. Take the flipper set and wait until your eyes are dark adapted. Then use trail and error to find out which correction (if any) works best. Warning, if you are not careful you may end up with an improper correction and this can lead to headaches. Take your time, may sure you allow your eyes to adjust.

To order a set, go to http://www.optego.com/. (Look for the “Night Myopia Diagnostic Flippers.”)

If you think correction is appropriate, you should discuss the situation with your eye care professional. Remember this may require the purchase of eyeglasses that will only be useful for naked eye nighttime observing; such eyeglasses are not necessary for binocular or telescope viewing. Whether such a purchase is worth the expense needs to be an individual decision.

For more information there is an article in Sky and Telescope magazine. See Joshua Roth. September 2005. “Special Report: Night Myopia.” Sky and Telescope. Vol 110, Number 3.

Photography

For information on taking photographs of astronomical objects, see “Simple Astrophotography” by Clayton Kessler.

Binoculars and Telescopes

After gaining experience with observing with the naked eye, the next step is to attempt observing the same objects with binoculars. Many objects that are barely visible with the naked eye reveal a lot of detail when observed through binoculars. In addition, binoculars increase the number of objects you can see.

After gaining experience with binoculars, you may feel ready to purchase a telescope. Keep in mind that many people who purchase a telescope on impulse, are subsequently unhappy with their instrument. To avoid this, take your time when deciding to buy a telescope. For more information, see Telescope Links.

References

For a list of the reference materials used to produce this guide, see the University Lowbrow Astronomer’s Book List.

Links

Copyright Info

Copyright © 2013, the University Lowbrow Astronomers. (The University Lowbrow Astronomers are an amateur astronomy club based in Ann Arbor, Michigan).
This page originally appeared in Reflections of the University Lowbrow Astronomers (the club newsletter).
This page revised Sunday, March 9, 2014 4:30 PM.
This web server is provided by the University of Michigan; the University of Michigan does not permit profit making activity on this web server.
Do you have comments about this page or want more information about the club? Contact Us.