J A N U A R Y
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Topical Science Supermoons, Blue Moon and a Lunar Eclipse In This issue What is a supermoon?
MoonMoon
What is a blue moon? How does a lunar eclipse occur? Why are there high tides with the full moon?
Supermoon Rising January 1st (Photo by Carol Jean Stalun via Flickr.)
This month of January 2018 is marked by some interesting lunar events. First of all, we had a so-called ‘Supermoon’ on January 1st. This occurs whenever the full moon happens to coincide with perigee, the point in the moon’s orbit when it comes closest to Earth. There will be another Supermoon on January 31st and this will also be a ‘blue moon’. When there are two full moons within the same calendar month, the event is known as a ‘Blue Moon’. not only that, but in some places, it will be possible to observe a total lunar eclipse on January 31st.
TOPICAL SCIENCE
JANUARY 2018
Perigee and Apogee; Supermoon and Micromoon The orbit of the moon around the Earth is not perfectly circular; it follows an elliptical path, with the Earth at one focus. Thus, the distance between Earth and moon varies considerably as the moon travels in its orbit around the earth. When the moon is at its closest position to Earth, it is said to be at Perigee and when it is farthest away, it is at Apogee. When Perigee coincides with a full moon, we have a ‘Supermoon’ and when the full moon occurs at apogee, then it is called a ‘micromoon’.
The picture on the left, provided by NASA. shows that the moon appears to be 14% bigger and 30% brighter when it is a supermoon, as compared to its apparent size and brightness as a micromoon.
The Full Moon & Spring Tides As most people are aware, tides are mainly caused by the gravitational pull of the moon. But the combined gravitational pull of the sun and moon on the Earth’s oceans causes particularly high tides when there is a full moon, because this is when sun, earth & moon are aligned. A high tide when the moon is full is called a ‘spring tide’. When the full moon is at perigee, there is an exceptionally high tide, since the moon is so close & gravity is an inverse square law. Unfortunately, the high spring tide, caused by the supermoon, coincided with the arrival of Atlantic storm Eleanor on the west coast of Ireland. This gave rise to serious flooding in places, as the wind drove the seawater onto the shore. Heavy rain made the situation even worse. The city of Galway suffered severe flooding, as shown in this picture, taken near the famous Spanish Arch. 2
TOPICAL SCIENCE
JANUARY 2018
The first full Moon of January is known as the ‘Wolf Moon’ ‘Wolf Moon’ is a traditional name for the first full moon of January. The name probably arose during a time when wolves roamed over northern Europe and North America and could be heard howling outside villages, looking for food by the light of the full moon, during the hungriest time of the year for them, when food was scarce. Other names that have been used, for obvious reasons, for the first full moon of the New Year are the Hunger Moon and the Snow Moon.
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TOPICAL SCIENCE
JANUARY 2018
BLUE MOON and LUNAR ECLIPSE We have already had a full moon on January 1st and the moon will be full again on January 31st. When two full moons occur within the same calendar month, the second one is known as a ‘Blue Moon’. It is a rather rare event, hence the expression “once in a blue moon”. It so happens that there will be a total lunar eclipse visible in some part of the world on January 31st. Unfortunately, the eclipse will not be visible from Ireland.
Diagram of Total Lunar Eclipse A lunar eclipse occurs at a full moon, when the sun, earth and moon are perfectly aligned. As the diagram shows, when the earth is between the sun and the moon, it casts its shadow on the moon. The darker shadow is called the ‘umbra’ and the lighter, outer shadow is the ‘penumbra’. We don’t have an eclipse at every full moon, because the moon’s orbit around the earth is not exactly in the same plane as the orbit of the earth around the sun, but makes a 5-degree angle with it. There are two points where the orbits intersect and these are known as ‘lunar nodes’. It is only when a full moon occurs near one of the lunar nodes that the sun, earth and moon are in a straight line and then a total eclipse is visible from those parts of the earth that are facing the moon at the time of the eclipse. There will be another Lunar Eclipse on the night of 27/28 July 2018 and this will be visible from Ireland. So make a note of it in your diary and watch out for it. About the Author: Margaret Franklin is a retired chemistry lecturer, who lives in the Republic of Ireland. For several years she published a column called ‘Topical Science’ in a local weekly newspaper, ‘The Westmeath Independent’. The aim of ‘Topical Science’ is to explain in plain language the science behind topical news items. Margaret was also a regular contributor to ‘Science Spin’ magazine, which unfortunately us now out of print.
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