Friends and family in social media were sharing an article claiming that on July 27th, 2018 Mars would be as large as the moon. This is a myth. There is a “blood moon” this weekend, and there will be something fascinating about Mars too. But if they appeared to be the same size, we would be in some serious trouble. Here is the science of what will happen with the moon and Mars this weekend.
On Friday, July 27th, a “blood moon” lunar eclipse will happen. The full moon passes through the shadow of Earth. What is special about this particular eclipse is that it will change the color of the moon to a reddish color. Obviously that is where the term “blood moon” comes from but, this one will happen for about 103 minutes. At the same time that the longest “blood moon” in history is happening, Mars will be only 35.9 million miles from Earth. According to experts, this is the closest that the “red planet” has been to Earth in 15 years. According to a story on the CBS News website,
For about half the world, the moon will be partly or fully in Earth’s shadow from 1:14 p.m. to 7:28 p.m. ET — six hours and 14 minutes in all. The period of complete eclipse — known as “totality,” when the moon appears darkest — will last from 3:30 p.m. to 5:13 p.m. ET. “Totality will last for 103 minutes, making it the longest eclipse of the 21st century!” said the Royal Astronomical Society in London.
So why does a total lunar eclipse like this cause the moon to look red? According to the National Aeronautics and Space Administration (NASA) website,
A total lunar eclipse happens when the whole moon enters Earth’s shadow. Some sunlight still reaches the moon, but first it goes through Earth’s atmosphere. The atmosphere filters out most of the sun’s blue light, so the moon looks red.
These optical processes are somewhat similar to what happens on Earth when we see orange-reddish sunsets and sunrises.
The NASA graphic below provides insight into why this particular event will be so long, relatively speaking. The moon will make a nearly perfect passage through the center of the umbra. The umbra is the darkest center portion of Earth’s shadow. Earthlings are familiar with the umbra. According to timeanddate.com, “every time the Sun goes down, we delve into the darkness created by Earth’s umbra. However, as with total solar eclipses, lunar eclipses only occur every so often because they require the Moon to enter the Earth’s umbra.” The Earth’s umbra causes total lunar eclipses, and the moon’s umbra causes total solar eclipses.
Total lunar eclipseNASA
This brings me back to the myth propagating around social media that the moon and Mars will be roughly the same size. It is clear that people are conflating the fact that we will experience a spectacular blood moon at the same time that Mars makes one of its closest passes to Earth. The aforementioned CBS story places the two events in proper context. Concerning the myth that the two bodies will be the same size, the story notes,
If that were true, we’d be in big trouble given the gravitational pulls on Earth, Mars, and our moon!” the NASA website states. Mars will more likely appear as a very bright star, and viewers will need no protective eye gear.
Mars has already gotten quite a bit of press this week. Earlier in the week, the European Space Agency announced that a radar system may have detected briny water body beneath the southern pole of the planet. It will get some more press this weekend, but the “blood moon” will actually be the star of the show.