The Solar Eclipse FAQ #1

An eclipse is a remarkable astronomical event that, even for a casual observer, inspires wonder and reflection to the very core.  The Western Hemisphere will be treated to a great show on August 21 as the Moon passes across the face of the Sun.  Many people throughout the United States will be treated to totality for several minutes.

What exactly is an eclipse?

An eclipse can typically be broken up into two types.  The first type is basically when an astronomical object is in the shadow of another object, such as a lunar eclipse -- The Moon is in the shadow of the Earth.  The second type is when an astronomical object passes in front of another, such as a solar eclipse -- The Moon passes in front of the Sun.

What is the difference between an annular eclipse and a total eclipse?  What makes 2017 so special?

The Earth orbits the sun in an elliptical path and the Moon orbits the Earth in an elliptical path -- they are not circles.  This causes the Sun and Moon to seem slightly larger or smaller depending where we or the Moon are in the orbit.  Think of a supermoon.  These occur when a Full Moon or New Moon is at perigee (closest approach to Earth).  See the image below (2) is the Moon at perigee and (3) is the Earth.

Moon at apogee (1), perigee (2) and the location of the Earth (3)

Also, the Sun is at its closest approach to Earth (perihelion) at the beginning of January and its farthest approach (aphelion) at the beginning of July.

As the Sun and Moon are visually very close to each other in angular size as seen from Earth we could have several situations.  When the Moon and Sun are further away from us (apo-) they will appear smaller.  However when they are closer to us (peri-) they will appear larger.

An annular eclipse occurs when the Moon is visually smaller than the Sun (due to some combination of the New Moon being closer to apogee and the Sun being closer to perihelion).  The Moon will not completely obscure the Sun.  See the image below.

Annular Solar Eclipse

A total eclipse occurs when the Moon is visually larger than the Sun (due to a combination of the New Moon being closer to perigee and the Sun being closer to aphelion).  The Moon will obscure the Sun.  See the image below.

Total Solar Eclipse

August 21, 2017 will be a total solar eclipse.  Aphelion occurs just over 6 weeks prior and the New Moon's perigee is a mere 3 days prior to the eclipse!  Great combination!

There is one other special type of eclipse that occurs around once every 160 years.  A hybrid eclipse is where the Sun and Moon are so very close in size that the eclipse changes from Annular to Total on different locations on the Earth.  The last one was on November 3, 2013 and you'll have to wait until 2172 for the next one!

The most common solar eclipse for everyone is the Partial Eclipse.  You are not in the zone of totality and will only see the Moon partially obscure the Sun.  Calgary will have 4 of these in the 2020's.  Mark your calendars for the evening of August 22, 2044!  Calgary will have a total solar eclipse!