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PHYS1160 APOD Assessment Isaac
SOLAR AND LUNAR
ECLIPSES
What’s all the
fuss about?
Hello there! My name is Dan, and I will be guiding you through a
series of questions relating to one (or two) of the most broadcasted
and unifying cosmic events that people enjoy from the comfort of
their own backyards – eclipses!
Often, when eclipses are reported on television, we are told very
little about what they are and how they occur. The reports are met
with many tens of thousands of eager people that flock to
observation areas to share in the moment when an eclipse occurs.
But why has there always been a widespread appreciation for the
event? Dating back to the Ugarit Eclipse of 1375BCE [1], eclipses
have been observed throughout most known human civilisations.
They confirmed Aristotle’s arguments that the Earth is round, and
contributed to proving Einstein’s Theory of Relativity [2]. What
exactly are eclipses and how many different types are there? How
can you best prepare to view one?
These questions, and many more, will be answered below.
Happy exploring!
PHYS1160 APOD Assessment Isaac Gunay Ada
Term 2, 2021 z5113412
2
To understand solar and lunar eclipses, we must first consider some basic facts regarding the three celestial
objects involved: Sun, Moon and Earth.
Nicholas Copernicus famously said “We revolve around the Sun like any other planet” [3]. This heliocentric
model of the solar system places the Sun at the centre and the Earth as one of the orbiting planetary objects.
It was then Johannes Kepler’s laws of planetary motion which matched astronomical observations of
planetary motion with heliocentrism [4]. It has since been understood that the Sun is positioned at the focus
of the Earth’s elliptical orbit. There is another elliptical orbit within Earth’s orbit: The Moon. And this time,
Earth is at its focus [5].
With these orbits within orbits, you should expect there to be times when the Sun, Earth and Moon align in
their positioning with respect to each other. If that is your expectation, you are correct!
To help us mentally visualise such phenomena, let us first consider the distances between these objects and
their sizes relative to each other.
At the centre of the solar system, the Sun is approximately 150 million kilometres away from Earth [6]
compared to 384,400 kilometres between Earth and the Moon [7]. The means that the Sun is positioned at
390 times the distance between Earth and the Moon.
Let’s start at the beginning
Not to scale
Not to scale
With respect to relative size, the Sun’s diameter is
approximately 109.1 times the diameter of the
Earth, with the moon’s diameter less than one third
the diameter of Earth [8].
Therefore, the apparent size of the
Sun from an observer on Earth
would be considered similar to
that of Moon because despite the
Sun being much larger than the
Earth, it is also much further away
when compared to the moon [9].
PHYS1160 APOD Assessment Isaac Gunay Ada
Term 2, 2021 z5113412
3
On the left of Figure 1 [22] is a digitally configured
image of a total lunar eclipse. The moon appears
reddish [10].
On the right of Figure 1 [22] is a digitally configured
image of a total solar eclipse. You might not be able
to tell, but Dan is wearing a pair of eclipse watching
glasses. To him, the moon appears very dark with a
ring of light emanating from around it.
LUNAR ECLIPSE SOLAR ECLIPSE
What is a lunar eclipse?
A lunar eclipse occurs when the Earth blocks the
Sun’s light from reaching and being reflected off the
Moon – in effect, the Earth’s shadow is being cast on
the Moon [11].
What is a solar eclipse?
A solar eclipse occurs when the Moon blocks the
Sun’s light from reaching Earth – in effect, the
Moon’s shadow is being cast on Earth [12].
The Sun’s light casts two shadows [13]:
Umbra (smaller shadow) and Penumbra (larger shadow)
What exactly am I looking at?
Although the Moon orbits the Earth once every 29.5 days [14], we do not see lunar
and solar eclipses occurring once every month. This is because of the Moon’s orbital
tilt: a 5 degree tilt of the Moon’s orbit from the Earth’s orbital plane [15]. It is for this
reason we see different types of lunar and solar eclipses.
The size of the Moon’s umbral shadow can
reach 250 kilometres in diameter when cast
on Earth’s surface [19]. This umbral shadow
zone generates a path as the Earth rotates:
this is called the path of totality [19].
Fig. 1: Solar
and Lunar
Eclipses
PHYS1160 APOD Assessment Isaac Gunay Ada
Term 2, 2021 z5113412
4
TYPES OF LUNAR AND SOLAR ECLIPSES
TOTAL Lunar Eclipse
A total lunar eclipse occurs when a full Moon
falls completely within Earth’s umbral shadow
[13]. Picture you are observing the Moon from
Earth:
The reddish appearance of the Moon is
caused by the Earth’s atmosphere refracting
the longer red wavelengths of the Sun’s light
toward the central umbral shadow region
[16].
PARTIAL Lunar Eclipse
A partial lunar eclipse occurs when only a part
of the Moon enters Earth’s umbral shadow
[13]. Picture you are observing the Moon from
Earth:
It is only the part of the Moon in the Earth’s
umbral shadow that will appear reddish [17].
PENUMBRAL Lunar Eclipse
A penumbral lunar eclipse occurs when the
Moon falls completely within Earth’s umbral
shadow [13]. Picture you are observing the
Moon from Earth:
These eclipses are difficult to spot, but you
should expect the Moon to simply darken
[18].
TOTAL Solar Eclipse
A total solar eclipse occurs when a full moon covers the
entire surface of the Sun [13]. You can observe this if you
are located within the path of totality.
Picture you are observing the Moon from Earth:
In the moments before the Sun is completely blocked by
the Moon, you will observe the phenomena known as the
diamond ring effect (see Fig. 2 [20]).