A BRIEF OVERVIEW OF SOLAR ECLIPSES

A BRIEF OVERVIEW OF SOLAR ECLIPSES

1. INTRODUCTION

A “Solar eclipse” is a celestial phenomenon where observers on the Earth see the Sun become dim when the Moon blocks out the light from the Sun totally or partially. When a solar eclipse occurs, the Moon is in its orbit between the Earth and the Sun such that the Sun, Moon and Earth are in a straight line. 

There are 3 main types of solar eclipses, according to their appearances (see Figure 1).

 

Figure 1: A diagram showing the different types of solar eclipses if observers are in different types of lunar shadows.

- An observer in position A: in the umbra -> sees a total solar eclipse

- An observer in position B: in the antumbra -> sees an annular solar eclipse

- An observer in position C: in the penumbra -> sees a partial solar eclipse

[Credit: user “Sigmanexus6” @ Wikipedia.org].

 

 

2. TYPE OF SOLAR ECLIPSES

2.1 A Total Solar Eclipse

These solar eclipses (Figure 2 and 3) occur when the Sun’s apparent size is smaller than the Moon’s apparent size.

 

Figure 2: Diagram shown the Sun-Moon-Earth configuration when a total solar eclipse occurs [Credit: user “Sagredo” @ Wikipedia.org].

 

Figure 3: The total solar eclipse in August 2008 above the Jiayuguan Fortress (part of the Great Wall of China) [Credit: REUTERS/David Gray].

 

2.2 An Annular Solar Eclipse

This type of solar eclipse (Figure 4) occurs when the Sun’s apparent size is greater than the Moon’s apparent size (see Figure 5), so at mid-eclipse the Sun looks like a ring of light.

 

Figure 4: An annular solar eclipse sequence from Utsunomiya city, Toshigi Prefecture, Japan (about 100 km north from Tokyo) in May 2012 [Credit: JIJI Press]. 

 

Figure 5: A comparison between the minimum and maximum apparent sizes of the Sun & the Moon. The Sun and the Moon have similar apparent sizes because the Sun is ~400 times larger than the moon but the Moon is ~400 times closer to the Earth than the Sun. 

 

But because of the elliptical shape of the Moon’s and the Earth’s orbit, the apparent size of the Sun and the Moon are not constant [Credit: user “Cmglee” @ Wikipedia.org].

 

2.3 A Partial Solar Eclipse

The moon only partially blocks out the light from the Sun, so the Sun appears like a crescent when a partial solar eclipse occurs (Figure 6).

 

Figure 6: A partial solar eclipse sequence from San Sebastian, Spain, in October 1996 [Credit: Juan Carlos Casado, TWAN].

 

2.4 A Hybrid Solar Eclipse

Besides the three main types of solar eclipse, there is a fourth type, called a “hybrid solar eclipse” or an “annular-total solar eclipse” (see Figure 7 and 8).

 

This fourth type of solar eclipse occurs when the lunar and solar apparent sizes are almost equal. 

 

The cone end of lunar umbral shadow does not contact the Earth’s surface during the early and late intervals of these phenomena (an annular solar eclipse) but contacts the Earth’s surface in the middle interval (a total solar eclipse).

 

This is caused by the curvature of the Earth’s surface and the elliptical shape of the Moon’s orbit.

 

Figure 7: A diagram showing the early interval of a hybrid solar eclipse. The surface of the side of the Earth faces away from the lunar umbral shadow’s cone, so the end of the shadow cone does not reach the Earth’s surface. An observer in the antumbra of the Moon (near the umbral cone end) sees an annular eclipse [Not to scale. Credit: http://moonblink.info/Eclipse/why/solar ].

 

Figure 8: A diagram showing the middle interval of a hybrid solar eclipse. The Earth’s surface at the noon side is closer to the Moon than the other side (because of the curved surface), along with the Moon’s orbit being closer to the Earth (because of the elliptical shape of the orbit). The umbral shadow end of the Moon reaches the Earth’s surface, so a total solar eclipse will occur [Not to scale. Credit: http://moonblink.info/Eclipse/why/solar ].

 

A solar eclipse can occurred any month if the lunar orbital plane is superimposed on the Earth’s orbital plane. But in fact, the lunar orbital plane is inclined about 5 degrees relative to the Earth’s orbital plane (see Figure 9). Because of this inclination there must be a “new moon” when the Moon is at the “lunar orbital node” (the point where lunar orbit cross the Earth’s orbital plane).

 

There must be a minimum of two solar eclipses each year (Figure 10), but in some years there is a maximum of five.

 

Figure 9: A diagram showing the 2 types of lunar orbital nodes (the ascending node & the descending node) and planes of the Earth’s and the Moon’s orbits [Source: http://astro.wsu.edu/worthey/astro/html/lec-celestial-sph.html ].

 

Figure 10: A diagram showing the case where if the Moon is far from the orbital node, a solar eclipse cannot occur, and the fact that a solar eclipse will occur six months after the previous one [Credit: Wadsworth Publishing Company/ITP]. 

 

3. OBSERVING SOLAR ECLIPSES

The methods for observing a solar eclipse when the Moon does not fully obscure the Sun consist of direct and indirect observation.

 

3.1 Direct Solar Eclipse Observation

 

Using the naked eye view the Sun via a filter or place a filter in front of the telescope. The types of filters that may be used for direct solar observing include:

 

- Black & White camera film (exposed to the light and developed film)

- A mylar filter

- A welding helmet filter/welder’s goggles (rated 14 or higher)

- A metallic glass solar filter

- An H-α filter

 

3.2 Indirect Solar Eclipse Observation

 

Figure 11: Point your telescope at the Sun, using shadow of the telescope to align it, and use a white projection card to show the solar image.

 

Figure 12:  Using the “pinhole telescope” concept, make a hole in some material and then use a projection card to show the solar image.

 

Figure 13: A tree also can make images of the Sun by the “pinhole telescope” concept because the gaps between the leaves behave like pinholes.

 

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WRITTEN BY

 

Pisit Nitiyanant

Astronomical Public Outreach Officer

Centre of Academic Affairs and Astronomy Information Services & History and Heritage Working Group

National Astronomical Research Institute of Thailand (NARIT)

 

The original version of this document was used for discussions at a meeting on Thai-Japanese Astronomy, Stargazing and Cultural Exchange held on 13 January 2015 at the Faculty of Humanities, Chiang Mai University, Thailand.