Lunar Eclipse

• Frequently Asked Questions About Lunar Eclipses
  • What is an eclipse of the Moon?
  • Which are the different phases of a Lunar eclipse?
  • What can you see during a Lunar eclipse?
  • What is the Danjon classification?
  • How can you observe a Lunar eclipse?
  • Where on Earth can you see a Lunar eclipse?
    • When were the past Moon eclipses visible from Spain?
    • When will next eclipses take place?
  • What are the Saros series?
    • More Information
  • What are the apogee and perigee?
  • What are the ascending and descending nodes?
  • Why can we see the Moon in an eclipse if it is inside of the Earth's shadow?
    • More Information
  • Why do we see the Moon reddish during eclipses?
    • More Information

What is an eclipse of the Moon?

The Moon, as well as all the other bodys of the Solar System except the Sun, has no light of its own but shines by sunlight reflected from its surface.

An eclipse of the Moon (or lunar eclipse) can only occur at Full Moon, and only if the Moon passes through some portion of the Earth's shadow. The shadow is actually composed of two cone-shaped components, one nested inside the other (see figure). The outer or penumbral shadow is a zone where the Earth blocks part but not all of the Sun's rays from reaching the Moon. In contrast, the inner or umbral shadow is a region where the Earth blocks all direct sunlight from reaching the Moon.

We have a partial Lunar eclipse when a portion of the Moon passes through Earth's umbral shadow. A total Lunar eclipse takes place when the entire Moon passes through Earth's umbral shadow.


The Sun, Moon and Earth positions during a Lunar eclipse. Earth's two shadows are the penumbra and the umbra.
Sizes and distances not to scale.

On certain ocasions, the Moon passes through Earth's penumbral shadow and this is what we identify as a penumbral Lunar eclipse.

Which are the different phases of a Lunar eclipse?

During an eclipse of the Moon, our satelite should cross, partly or totally, the penumbra and/or the umbra. There are a few moments of special interest that are the following:

• First penumbral contact (P1): Moon begins to pass through the Earth's penumbral shadow. Beginning of the Eclipse and of the penumbral phase.
• Second penumbral contact: Moon entirely inside the Earth's penumbral shadow.
• First umbral contact (U1): Moon begins to enter the Earth's umbral shadow. End of the penumbral phase and beginning of the first parciality phase.
• Second umbral contact (U2): Moon entirely inside the Earth's umbral shadow. End of partility phase and beginning of totality.
• Third umbral contact (U3): Moon begins to leave the Earth's umbral shadow. End of Totality. Beginning of the second partiality phase.
• Fourth umbral contact (U4): Moon leaves completely the Earth's umbral shadow. End of partiality phase and beginning of second penumbral phase.
• Third penumbral contact: Moon begins to leave the Earth's penumbral shadow.
• Fourth penumbral contact (P4): The Moon exits the Earth's penumbral shadow. End of the penumbral phase and end of the Eclipse.

Codes used in the figures for eclipses from the NASA and in Serviastro pages are shown, between brackets, in previous list.

Part of the technical file for the Lunar Eclipse of 3 March 2007.
Taken from F. Espenak, NASA's GSFC - 2006 April 20
http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html

What can you see during a Lunar eclipse?

During a lunar eclipse, you can see the Moon entering the penumbra region, when the disc becomes a little darker. During a penumbral lunar eclipse, only the most expert observers can notice the eclipse. If the eclipse progresses further, the Moon starts to enter the umbra. During this phase you can see that a part of the lunar disc is dark, while the other part is still illuminated. If the eclipse is a partial one, this is all you will see. On the contrary, if the eclipse is a total lunar eclipse, the Moon will be increasingly covered by the Earth's shadow until the entire lunar disc is in the umbra. At this moment, the eclipse is at the totality phase, but with a much fainter and redder aspect.

What is the Danjon classification?

The French astronomer André-Louis Danjon, published a table to classify the Moon eclipses according to the brightness during the total phase. This table sorts the lunar eclipses into five different degrees:

• L=0 : Very dark eclipse. The Moon is almost invisible.
• L=1 : Dark eclipse. The Moon turns grey or brown. The surface details can hardly be distinguished.
• L=2 : Dark red eclipse, with the central zone very dark and somewhat brighter edges.
• L=3 : Reddish eclipse with, usually, bright colours at the edge of the umbra.
• L=4 : Very bright eclipse, with bright red or even orange colours. Usually, the edges are very bright with blueish colours.

How can you observe a Lunar eclipse?

Lunar eclipses are one of the easiest and most beautiful astronomical phenomena to observe. You have just to find a confortable place from where the Moon can be seen. Wearing warm clothes, having some coffee and something to eat is highly recomended if you are outdoors. Once everything is ready you just have to wait for the show to start (far more interesting that some TV programs). To see the event in detail, you can also use a small telescope or binoculars. This way you can watch the umbra go across the lunar craters.

Where on Earth can you see a Lunar eclipse?

A lunar eclipse can be seen from any place on Earth where night¹ occurs at the same time as the eclipse. For this reason, and in contrast with solar eclipses, a lunar eclipse can be seen from over half of the Earth surface.

1_ What is really important is that the Moon should be over the horintzont. Moon eclipses can only happen with Full Moon, then to say that should be night or that the Moon should be ober the horizont is equivalent. Full Moon always rise with the sunset.

When were the past Moon eclipses visible from Spain?

• 28 July 2018: Total eclipse visible from the beginning of totality.
• 7 August 2017: Partial eclipse from Spain.
• 10-11 February 2017: Penumbral eclipse. Some images available on ServiAstro.
• 28 September 2015: Total eclipse visible in all phases. Live-Broadcasted by ServiAstro. Check the images.
• 15 June 2011: Total eclipse. The Moon rose completely eclipsed.
• 21 December 2010 : Total eclipse visible from Spain up to the beginning of the totality.
• 31 December 2009 : Partial eclipse.
• 6 August 2009 : Penombral eclipse enterely observable.
• 16 August 2008 : Partial eclipse, almost visible from the beginning.
• 21 February 2008: Total eclipse, enterely visible. Live-Broadcasted by ServiAstro. Check the images.
• 3 March 2007: Total eclipse, enterely visible. Live-Broadcasted by ServiAstro. Check the images.

When will next eclipses take place?

• 21 January 2019: Total eclipse visible in almost all its phases.
• 26 May 2021: Total Eclipse not visible from Spain.
• 19 November2021: Parcial Eclipse.
• 16 May 2022: Total Eclipse visible in almost all its phases.
• 22 November 2022: Total eclipse not visible from Spain.
• 5 May 2023: Penumbral eclipse almost invisible from Spain.
• 8 October 2023: Partial eclipse visible from Spain.

What are the Saros series?

The periodicity and recurrence of eclipses is governed by the saros cycle, a period of approximately 6,585.3 days (18 years 11 days 8 hours) separates two very similar eclipses. It was known to the Chaldeans as a period when lunar eclipses seem to repeat themselves, but the cycle is applicable to solar eclipses as well. Not all eclipses within the same series are very similar. Only two consecutive eclipses resemble each other. The eclipses within one series begin as penumbral eclipses, later they become partial eclipses, then total eclipses, then partial eclipses again and finally penumbral eclipses. After the last penumbral eclipse, the series concludes. The whole proces can last for over 1500 years.

More Information

1. Short article covering basic concepts.
2. Webpage with explanations from the NASA Eclipse Home Page
3. Detailed explanation with figures about the cycles of eclipses.
4. Detailed exposition of the lunar months , ending with the concept of Saros series.

What are the apogee and perigee?

Apogee and perigee are two especial points of the Moon's orbit, and of all the other bodys that orbit around the Earth, like artificial satelites,

The Moon's orbit around our planet is not a perfect circle, but an ellipse. At one end of the ellipse (called apogee) the Moon lies 406,700 km from Earth. At the other end (called perigee) the Moon is only 356,400 km away -- a difference of 50 thousand km!

Those extreme points of any elliptic orbit are generically called apsides independently of the central body.

The notable difference of distances to the Earth between apsides affects also to the apparent size of the Moon that is also noticeable.

What are the ascending and descending nodes?

Earth's orbit around the Sun and Moon's orbit around the Earth are not in the same plane, but: around 5º.

Lunar nodes (Moon's orbit nodes) are the points in the ecliptic plane (where Earth and Sun are located) where the orbit of the Moon crosses it. The ascending node is where the Moon crosses to the North of the l' ecliptic. The descending node is where it crosses to the South of the ecliptic.

These two points are important. Eclipses occur only near to the lunar nodes: Solar eclipses occur when the passage of the Moon through a node coincides with the New Moon; lunar eclipses occur when it coincides with the Full moon.

Why can we see the Moon in an eclipse if it is inside of the Earth's shadow?

We are able to see the Moon during the totality phase even if the Earth is blocking the light from the Sun, part of this light can still reach the Moon. That is due to the atmosphere of the Earth that deviates some light rays towards the umbra. This light rays reach the surface of the Moon and get reflected again to the Earth.

The process in which the light rays get deviated from its straight way throught the terrestrial atmosphere is a particular case of a phenomenon called refraction.

More Information

1. General webpage about refraction
2. Another important phenomenon consequence of the atmospheric refraction.

Why do we see the Moon reddish during eclipses?

In fact, even if red is the most usual colour, the Moon has not always the same colour nor the same tone. Every eclipse appears differently. That is the reason why the French astronomer André-Louis Danjon proposed the brightness scale of the Moon during totality phase.

Centre of the Eclipse on 6 July 1982
Copyright 2007, Fred Espenak, www.MrEclipse.com

Centre of the Eclipse on 20-21 January 2000.
Copyright 2007, Fred Espenak, www.MrEclipse.com

Centre of the Eclipse on 16 July 2000.
Copyright 2007, Fred Espenak, www.MrEclipse.com

This colour of the Moon during totality is due to the fact that the light that reach the surface of the Moon has passed through the terrestrial atmosphere. The atmosphere not only desviates the light rays, but also afects in a different way to each colour: red gets through more easily than blue.

The most important phenomena taking part in this process are Rayleigh dispersion and Miedispersion, same phenomena that explains why the sky is blue, the Sun yellow when is hight up in the sky and reddish when is down -rising or setting- or why the clouds are white.

More Information

1. Index of contents about atmospheric optics in Hyperphysics
2. Webpage describing atmospheric phenomena caused by Rayleigh and Mie dispersions (Hyperphysics)
3. Webpage about other related atmospheric phenomena (Hyperphysics)