• Skip to main content
  • Skip to primary sidebar

TelescopeAdviser.com

  • TELESCOPES
  • BINOCULARS
  • ACCESSORIES

Astronomy

Total Solar Eclipse 2017

Last updated: July 6, 2017

Photo by Damien Deltenre

Something strange will happen on Monday August 21, 2017. Daylight will turn into darkness, temperatures will drop, the sound of crickets will replace birds chirping, and millions of people will be staring up at the sky. For the first time in 99 years, a total solar eclipse will sweep across the entire United States along a narrow corridor from Oregon to South Carolina. The “Great American Eclipse” will be one of the most beautiful natural phenomena you can experience.

The last total solar eclipse in the United States occurred in 1979 and was only visible from states in the northwest corner of the country (Washington, Oregon, Idaho, Montana, and North Dakota).

What is a total solar eclipse?

Photo by: Renata3

A total solar eclipse occurs when the moon completely blocks out the sun. As the moon passes between the Earth and sun, it casts two types of shadows on the Earth. The umbra is the moon’s innermost and darkest shadow. If you’re in the path of the moon’s umbral shadow you will see the sun’s entire disk obscured and the glowing appearance of the sun’s corona, its outer atmosphere. This phase of the eclipse is called totality. The narrow track of the umbra moving across the U.S. is known as the path of totality and will be only 71 miles at its widest point.

The penumbra is the moon’s faint outer shadow. If you’re within the moon’s penumbral shadow, you will see a partial solar eclipse. The penumbra covers a much larger area on Earth allowing the partial eclipse to be visible throughout North America including Canada as well as the northern part of South America, and the western parts of Europe and Africa.

A total solar eclipse occurs somewhere on Earth about once every 18 months. This may sound like a more common occurrence than one would think but due to the narrow path of totality, the odds of viewing a total solar eclipse from a particular location on Earth are rare.

There are other types of solar eclipses but nothing compares to a total solar eclipse. An annular solar eclipse occurs when the Earth, moon and sun are properly aligned but the moon is too far away from Earth due to its elliptical orbit. The moon’s smaller apparent size is unable to completely cover the sun and it appears as a very bright ring. If the alignment of the Earth, moon and sun is off slightly, only a portion of the sun’s disk will be covered by the moon. This is a partial solar eclipse and it is not nearly as dramatic or spectacular as witnessing totality even if it covers 99 percent of the sun.

What you will see during a total solar eclipse

Several notable events occur just prior to totality. As the moon almost completely covers the sun, beads of light appear along the edge of the moon as sunlight shines through the lunar mountains, valleys and craters. This effect is known as Baily’s Beads. When only a single large bead of light is visible, it takes on the appearance of a shining diamond ring.

Once totality is reached, the sun’s normally hidden corona becomes visible. The corona is an aura of plasma that is millions of degrees hotter than the surface of the sun and extends millions of miles into space. Witnessing the corona is a profoundly intense experience that is simply beyond words.

As the sky grows dark, the blue skies began to dull and turn deeper blue to twilight blue to blue-black. The darkened skies allow the brighter stars and planets to become visible including Venus and Jupiter. Looking to the horizon you’ll see orange and yellow twilight in all directions. You’re actually seeing light from beyond the umbral shadow in areas where the eclipse is not in totality.

How long totality will last varies depending on your location and how close you are to the center of the umbra. The longest duration of totality will be 2 minutes and 40 seconds.

Where to see the total solar eclipse

The “Great American Eclipse” will first appear in Oregon and cross Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina. Very tiny corners of Montana and Iowa are also just inside the path of totality.

NASA’s Total Solar Eclipse Interactive Map plots the path of totality on Google Maps. Zoom in and click anywhere on the map to see the start and end times, duration, and magnitude of the solar eclipse at a specific location.

A large portion of the U.S. will be within a day’s drive of seeing the total solar eclipse. Several cities and towns have been preparing for this day for several years. Millions of people are expected to witness this event and populations are anticipated to at least double in some locations. Many hotels along the path of totality have been booked for over a year in advance. Expect major traffic getting to and leaving your viewing site.

The best locations to view the longest duration of totality are in Missouri, Illinois and Kentucky along the middle of the path of totality. Just outside of Carbondale, Illinois will be the point of Greatest Duration and an area near Hopkinsville, Kentucky will be the point of Greatest Eclipse. Both are expected to see up to 100,000 visitors. Hopkinsville has been promoting itself as “Eclipseville”.

How to safely view the total solar eclipse

Never look directly at the sun without proper eye protection. You can severely damage your eyes and go blind. Even staring at 1 percent of the sun (a 99 percent partial eclipse) or Baily’s Beads is enough to damage your eyes. Sunglasses are not a sufficient or acceptable form of eye protection, not even wearing two pairs.

Direct observation of the sun during a total solar eclipse can be done safely if using a certified solar filter that meets ISO Standard 12312-2. A solar filter will protect your eyes from the sun’s ultraviolet and infrared radiation, and intense visible light. Certified solar filters can be found in glasses, goggles and hand-held solar shields or viewers. Low cost solar eclipse glasses with cardboard frames can be purchased in packs of 5 or 10. More durable plastic frame versions are also available.

Always inspect your solar filter before use. If it is has a tiny hole, is scratched or damaged in any way, throw it in the trash. Make sure to put your glasses on first before before looking up at the sun. Only once the moon has completely covered the sun is it safe to remove your glasses and view the total eclipse with the naked eye. The instant the sun reappears, immediately return to your solar eclipse glasses or solar viewer to watch the end of the eclipse. At some locations, a person may make an announcement or blow a whistle to indicate when to remove your glasses and when to put them back on.

An alternative form of eye protection is number 14 welding glass, the only shade dark enough for solar viewing. Welding glass of an unknown or lower number is not safe to use. Stacking different numbers of welding glass together does not provide the same protection as number 14 welding glass.

If you choose to watch the eclipse through a telescope or binoculars, or want to take photos with a camera, you must have a proper solar filter attached to the end optical device or you can permanently damage your eyes. Do not use a solar filter that attaches to the eyepiece.

Another method to view the partial solar eclipse phase is indirectly with a pinhole camera. You can make a simple pinhole camera to project the image of the sun onto a surface and watch the changing crescent shape of the sun. Poke a pinhole through the center of one piece of white cardboard. With your back to the sun, allow the sunlight to shine through the pinhole onto a second piece of white cardboard held in front of you. The projected image will be upside down. Moving the two pieces of cardboard further away from each other will enlarge the image. Moving them closer together will brighten the image. Do not look at the sun through the pinhole.

When is the next total solar eclipse?

If you miss this year’s solar eclipse, the next total solar eclipse visible from the United States will occur on April 8, 2024. The path of totality will travel from Texas to Maine with a much longer duration of totality, almost 4.5 minutes.

Be warned, many people who witness a total solar eclipse for the first time become addicted and start to plan trips around the timing of the next solar eclipse. These passionate eclipse chasers will plan years in advance and are willing to go anywhere in the world just to catch another glimpse of totality.

Glossary of Astronomy Terms

Last updated: July 1, 2017


You’ve probably seen or heard many unfamiliar terms while researching telescopes. We’ve included the definitions of some of the most commonly used words in astronomy as a quick and easy reference.


A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


Altazimuth Mount

A type of telescope mount that moves up and down (altitude), and side to side (azimuth) similar to a camera tripod.

Altitude and Azimuth

The two positions in the horizontal coordinate system. Altitude refers to the angle between the observer’s horizon and the object. Azimuth refers to the angle of the object along the horizon measured from north to east.

Aperture

Aperture is the diameter of the telescope’s light-gathering lens or mirror. The larger the aperture, the more light will be collected resulting in brighter and sharper images. Aperture is typically measured in millimetres or inches.

Apparent Field of View

The angular diameter (in degrees) of the image viewed through the eyepiece. It is a property of the eyepiece.

Apparent Magnitude

A number that is the measure of the brightness of a celestial object to the naked eye. The lower the value, the brighter the object appears.

Asteroid

A minor planet orbiting the Sun with a diameter greater than one metre and mainly composed of minerals and rock.

Astigmatism

A type of optical aberration where light does not come to focus on a single plane. Most telescope designs do not suffer from noticeable astigmatism.

Astronomical Unit (AU)

The average distance between the Earth and the Sun used to measure distances within our Solar System. One AU is about 93 million miles or 150 million kilometres.

Astronomy

The study of celestial objects and phenomena.

Astrophotography

Taking photographs of celestial objects and phenomena.

Aurora

A natural light display in the sky produced by the interaction of charged particles when solar wind disturbs the magnetosphere. Known as Aurora Borealis in the Northern Hemisphere and Aurora Australis in the Southern Hemisphere.

Averted Vision

Viewing an object by looking slightly off to its side and not directly at it. This allows fainter details to appear.


Barlow Lens

A lens used along with an eyepiece that increases the telescope’s focal length and the magnification of the eyepiece. A 2x Barlow would double the eyepiece’s magnification.

Binary Star

A system of two stars that orbit around their common center of mass. Also known as double stars.

Binoculars

An optical device with a lens for each eye used to view distant objects.


Catadioptric Telescope

Also known as a compound telescope. Uses a hybrid design with a combination of lenses and mirrors in a sealed tube which fold the optical path to form an image. This provides a focal length much longer than the length of the compact optical tube. They are more commonly known by the two popular designs: Schmidt-Cassegrain and Maksutov-Cassegrain.

Celestial Object

A natural object that exists in the Universe and is located outside of Earth’s atmosphere.

Celestial Sphere

An imaginary sphere with a large radius and the Earth located at its center. An observer can view celestial objects as though they are projected on to the inner surface of the celestial sphere.

Chromatic Aberration

A type of optical aberration present in some refractor telescopes. When light passes through the objective lens, each wavelength of light is refracted by a different amount and the colors fail to converge at the same focal point. Think of the rainbow effect of light passing through a glass prism. Chromatic aberration is typically seen as color fringing around bright objects, like a purple halo around planets and stars.

Collimation

The process of manually adjusting the mirrors of a telescope when they are out of alignment.

Coma

A type of optical aberration (comatic) present in Newtonian reflector telescopes. With coma, stars appear distorted near the edge of the field of view while stars in the center of the field of view are unaffected.

Comet

A large ball of ice and rock that orbits the Sun. When a comet passes close to the Sun, it releases gas forming a coma (gas cloud) and a tail. Comets can range in size from hundreds of metres to tens of kilometres in diameter.

Compound Telescope

See ‘Catadioptric Telescope’

Computerized Telescope

A telescope on a GoTo mount with a small motor drive, built-in computer and hand-held controller which allows you to look up celestial objects in a database. The GoTo will automatically locate the specific object and track its movement across the sky.

Conjunction

When two planets or celestial objects are aligned so that they have the same right ascension and appear to be close together as observed from Earth. It is caused by the perspective of the observer as the two objects involved are not actually close to each other in space.

Constellation

A recognizable grouping of stars that form a pattern. There are 88 constellations which are typically named after animals, mythological characters or objects.

Cooldown Time (Thermal Stabilization)

The time required for a telescope’s optics to be brought to ambient temperature for optimal viewing.


Dark Adaptation

The adjustment of the eyes to darkness or low light conditions. Dark adaptation is necessary to see faint objects.

Declination

One of two angles used to locate a point on the celestial sphere. It is comparable to geographic latitude.

Deep-Sky Objects (DSO)

Celestial objects that are not individual stars and are outside of our Solar System such as star clusters, nebulae and galaxies.

Dobsonian

A simplified altazimuth mount for Newtonian reflectors. The mount sits on the ground, swivels 360 degrees and moves up and down.

Double Star

See ‘Binary Star’


Eclipse

When a celestial object like a moon or planet temporarily moves into the shadow of another celestial object. A Lunar Eclipse occurs when the Earth blocks the sun and the Earth casts a shadow on the moon. A Solar Eclipse occurs when the moon blocks the sun and the moon casts a shadow on the Earth.

Ecliptic

The circular path on the celestial sphere that the Sun appears to follow over the course of a year.

Equatorial Mount (EQ)

A type of telescope mount that is more precise than an altazimuth mount and better for astrophotography. Aligning one axis with Polaris allows you to adjust only the polar axis to track an object as the Earth rotates.

Equinox

One of two times per year when the Sun crosses the plane of the Earth’s equator causing day and night to be of equal length.

Exit Pupil

The width of the cone of light that exits the eyepiece at the exact eye relief distance.

Eyepiece

The part of the telescope that you look into. They are typically 1.25″ or 2″ in diameter. To change the telescope’s magnification, you can switch eyepieces with different focal lengths.

Eye Relief

The distance between the eyepiece lens and the observer’s eye while still being able to see the entire field of view. Outside of this distance, the observer will see a reduced field of view.


Field of View (FOV)

See ‘Apparent Field of View’ and ‘True Field of View’

Filter

An accessory that attaches to the bottom of an eyepiece to enhance details of celestial objects. Neutral density or polarizing filters can reduce glare from bright objects like the moon. Color filters can enhance specific planetary details by blocking certain wavelengths of light.

Finderscope

An optical device attached to a telescope used to locate a celestial object and aim the telescope in its direction.

Focal Length

The distance from the objective lens or mirror to the focal point.

Focal Point

The point where parallel rays of light meet after passing through a lens or reflecting off of a mirror.

Focal Ratio (f/ratio)

Equal to the focal length of a telescope divided by its aperture size. A telescope with a focal length of 1200mm and an 8″ (203mm) aperture would have a focal ratio of f/5.9.

Focuser

A device attached to the telescope into which an eyepiece is inserted. It can be adjusted to bring the image viewed through the telescope into focus. Types of focusers include helical, rack and pinion, and Crayford.


Galaxy

A system of stars, gas, dust, and dark matter that is gravitationally bound.

Globular Cluster

A spherical collection of stars that is tightly bound by gravity and has a high stellar density towards its center. They are found orbiting in the outer regions of a galaxy. Globular Clusters are much older and contain more stars than Open Clusters.

GoTo

See ‘Computerized Telescope’


Inclination

The angle between a reference plane and the orbital plane of an orbiting object.


Light Pollution

Brightening of the sky caused by artificial light which can significantly reduce the ability to observe celestial objects.

Light Year

The distance that light travels in one year, used to express astronomical distances. Equal to about 5.9 trillion miles or 9.5 trillion kilometres.


Magnification

Equal to the focal length of a telescope divided by the focal length of an eyepiece. For example, a telescope with a focal length of 1200mm and an eyepiece with a focal length of 10mm would provide a magnification of 120x. Also known as power.

Maksutov-Cassegrain

A type of catadioptric telescope design that uses a combination of a spherical mirror and a full aperture meniscus lens to correct for spherical aberration. Also known as a Mak.

Messier Objects

A catalog of 110 deep-sky objects including nebulae, open clusters, globular clusters and galaxies. The initial list was created in 1771 by French astronomer Charles Messier.

Meteor

A streak of light caused by a meteoroid, comet or astroid entering the Earth’s atmosphere at rapid speed causing it to heat up. Also known as a shooting star.

Meteoroid

A small rocky or metallic body in outer space ranging in size from a small grain to a width of 1 metre.

Meteor Shower

A number of meteors appearing to originate from the same point in the sky. They occur at particular dates each year when the Earth passes through streams of cosmic debris.

Milky Way

The galaxy that contains our Solar System.

Mount

A mechanical structure that supports a telescope which allows it to point in the direction of celestial objects. The two types of telescope mounts are Altazimuth and Equatorial.


Nebula

A cloud of gas and dust in outer space. Nebulae (plural) are visible as bright patches in the night sky or as opaque clouds that block light from luminous objects behind them (dark nebula).


Objective

A telescope’s main light-gathering lens or mirror.

Open Cluster

A loose grouping of stars found in the disk of a galaxy. They can contain up to a few thousand stars. Open Clusters are younger and less dense than Globular Clusters.

Optical Tube Assembly (OTA)

The main tube of a telescope which contains the optics (objective lens or primary mirror). It does not include the mount.

Outer Space

The Universe beyond Earth’s atmosphere.


Parabolic Mirror

A reflective surface with a concave shape designed to bring light to focus at a single point.

Planisphere

A circular star map that can be adjusted to show the stars and constellations as they would appear at a specific date and time.

Polaris

The brightest star in the constellation Ursa Minor. It is very close to the celestial north pole which is why it is commonly referred to as the North Star. It is used for navigation as it appears to remain stationary as the Earth rotates.

Primary Mirror

The main light-gathering mirror of a reflector telescope.

PushTo

A form of computerized telescope with a hand-held controller which allows you to look up a celestial object in a database. After selecting your desired object, the controller will provide directional arrows for you to move the telescope by hand until it is in the correct position. Unlike a GoTo, a PushTo requires manual tracking.


Red Dot Finder

A reflex sight finder that doesn’t provide any magnification. Look through the red dot finder’s viewing window and align the red LED dot with the celestial object you wish to observe by moving the telescope.

Reflector Telescope

A type of telescope that is open at one end and uses a concave primary mirror to collect and focus incoming light onto a flat diagonal secondary mirror which reflects the image to an eyepiece on the side of the optical tube.

Refractor Telescope

A type of telescope that has a convex objective lens at the front end of a sealed tube and an eyepiece at the rear end. Refractors were the first telescopes and the image most people have when thinking of the word “telescope”.

Right Ascension

The angular distance measured eastward along the celestial equator. It is the celestial equivalent of geographic longitude.


Schmidt-Cassegrain

A type of catadioptric telescope design that uses a combination of a spherical primary mirror, a convex secondary mirror and a corrector plate (aspheric lens). Also known as an SCT.

Solar Filter

A filter placed at the front of a telescope when viewing the Sun to block most its light. They are typically made from glass or plastic film. Failure to use a solar filter when your telescope is pointed at the Sun can cause severe eye damage and blindness.

Solar System

The system of eight planets and their moons as well as dwarf planets, comets, asteroids and meteoroids that orbit the Sun.

Solstice

The two times of the year when the Sun is at its most northerly or southerly point in the sky.

Spherical Aberration

A type of optical aberration where a spherical mirror is unable to focus light to a single point. It can be eliminated by using a parabolic mirror.

Spiral Galaxy

A type of galaxy structure that has spiral arms extending from the galaxy’s center.

Star

A luminous sphere of gas held together by its own gravity. Thermonuclear fusion of helium and hydrogen in its core produces energy. The Sun is the closest star to Earth.

Star Cluster

A grouping of stars which are gravitationally bound. See ‘Globular Cluster’ and ‘Open Cluster’.

Star Diagonal

An accessory consisting of an angled mirror or prism that fits in the telescope’s focuser and accepts an eyepiece. It allows a more comfortable viewing angle especially when the telescope is pointed at or near the zenith.

Star Hopping

A technique for locating faint celestial objects by using bright stars as a reference and a star chart.

Sunspot

A temporary dark spot that appears on the surface of the Sun. A sunspot has a reduced surface temperature compared to its surroundings. Sunspots can be viewed with a telescope using a proper solar filter.

Supermoon

Occurs when a full moon is also at its closest distance to Earth along its orbit.

Supernova

A dying star that experiences a massive explosion which expels most of its mass. It appears as a very bright star before it fades away.


Telrad

A reflex sight finder that doesn’t provide any magnification. It projects three red LED concentric circles onto the viewing window. This helps when star hopping as the circles aid in judging the angular distances between objects.

Tracking

Moving the telescope in small increments as the earth rotates to keep the celestial object within the field of view.

Transit

When a celestial object appears to move across the surface of another celestial object, blocking a small part of it. Typically seen as Mercury or Venus transiting the Sun.

True Field of View

The angular size (in degrees) of the amount of sky that can be seen through an eyepiece when used with a telescope.


Zenith

The imaginary point in the sky directly above the observer.

Zodiac

The area of the sky centered on the ecliptic along which the Sun, moon and most of the planets move over the course of a year. It is divided into 12 parts with each part named for a nearby constellation.

Primary Sidebar

Meade Infinity 102mm AZ Refractor Telescope Review

Orion SkyQuest XT6 Classic Dobsonian Telescope Review

Orion StarBlast 6 Astro Reflector Telescope Review

Best Telescopes for Kids

Best Budget Telescope

Best Telescope For Beginners

Best Astronomy Books For Beginners

Copyright © 2023 · TelescopeAdviser.com

Privacy Policy · Affiliate Disclosure · Contact Us