In an unusual astronomical coincidence, the 21st century’s longest total lunar eclipse will occur on the same night as Mars’s opposition, and both reddish objects will appear in the same part of the sky.
A planet is described as being in opposition when it and the Earth are on opposite sides of the Sun. Full Moons occur when the Moon is in opposition to the Earth. Objects in opposition rise at sunset and remain visible all night, then set at sunrise.
This particular Mars opposition is occurring when the Red Planet is closest to the Earth, so Mars will be at its largest apparent size when viewed through a telescope and will also be significantly brighter than usual.
Both the lunar eclipse and the Mars opposition will occur on Friday, July 27. Just four days later, Mars will be closest to the Earth, at a distance of 35.8 million miles (57.6 million km). An opposition in which a planet is also at its closest point to the Sun is known as a “perhihelic opposition.”
Because the Red Planet has an elliptical orbit, its distance from Earth ranges from a minimum of 34.6 million miles (55.8 million km) to a maximum of 140 million miles (225 million km). In 2003, the two planets came within 34.6 million miles of each other, the closest they had been in 60,000 years.
Friday’s total lunar eclipse will be visible to observers in Africa, the Middle East, southern Asia, Europe, South America, and Australia. Its total phase, during which the Moon appears red or orange, will last an hour and 43 minutes. From beginning to end, including the partial phases, the entire eclipse will last for almost four hours.
The Moon’s reddish color during the total phase of an eclipse is caused by the scattering of sunlight through Earth’s atmosphere. Because the atmosphere does more scattering of shorter light wavelengths, such as green and blue, longer wavelengths, such as red, become most visible.
During this particular eclipse, the Moon will travel through the center of Earth’s umbra, the darkest part of its shadow. It will also be at apogee, the furthest point in its orbit from the Earth, and will therefore take longer to pass through Earth’s shadow. These two phenomena are the reasons why this eclipse will be so long.
Slooh will also host a live cast on the Mars opposition on Thursday night, July 26. Unlike the lunar eclipse, Mars, like the ordinary full Moon, will be visible all night everywhere in the world barring obscuration by clouds.