On the night of September 27 the moon will be putting on quite a show for the northern hemisphere. We will have a harvest moon, blood moon, and Supermoon eclipse! One might wonder if this happens once in a blue moon but the answer to that is no. This is still quite a rare event – there have only been 5 Supermoon eclipses since 1900.
The Harvest Moon is the full moon closest to the autumn equinox, which is around September 20-21. This means the Harvest Moon is usually in September but occasionally falls in October.
Autumn full moons are special because the path of the moon (and sun and planets too) makes a narrow angle in the sky so they travel closer to the horizon and further to the north. With the moon low on the horizon it appears much larger than when it is higher in the sky. This is actually an optical illusion and the subject of another post but you can learn more here.
A Blood Moon is a relatively new term, which refers to the moon during a full eclipse that is part of a lunar tetrad. A lunar tetrad is four total eclipses of the moon spaced 6 months apart with no partial eclipses in between. As you can imagine this does not happen very often. There are a total of 8 tetrads in this century. The September 27 Blood Moon is the final eclipse in the tetrad that began a year and a half ago.
Supermoons occur because the moon’s orbit around the Earth is not circular but is slightly stretched into an eclipse. This means that at different points in its orbit the moon may be closer to the Earth than at other points. When the close orbit occurs during a full moon, the moon appears slightly larger and is called a Supermoon. The points where the moon is furthest away gives us a minimoon. The difference between these two extremes can be as much as 14% larger and 30 times brighter!
An eclipse occurs when the path of the moon around the Earth is such that the Earth passes directly between the sun and moon and its shadow covers the moon. A full eclipses occurs fairly often. Any particular location on the Earth can expect to see an eclipse every 2.5 years.
While the September 27 Supermoon eclipse is rare, it is not a blue moon. According to folklore, a blue moon occurs when there are two full moons in a calendar month. The last blue moon was July 31, 2015 and the next will be January 31, 2018.
Want more moon facts? Check out the NASA Animations and Visualizations Gallery.
Between the Harvest moon and the Supermoon phenomena, the moon is going to appear quite large in the sky on the night of September 27. Here is a simple way to objectively observe the size of the moon.
First, gather a bunch of straws of various sizes – coffee stirrer, drinking straw, milk shake straws and anything else you can find.
Then, when the moon is full and in the sky, look at the moon through the straws.
- Which straw best fits the moon’s size?
- Is the size different when the moon is low on the horizon compared to high in the sky?
- Draw a picture of how the moon looks through the straws and observe the moon through out the year. How does its size change?
Want to further explore the size of the moon? Graph 3 pounds of play dough and try this activity from the Astronomy from the Ground Up program.