Tomorrow's harvest supermoon will also be a partial lunar eclipse. How to best view the rare celestial event.

On Tuesday, Sept. 17, the harvest supermoon will coincide with a partial lunar eclipse, offering a unique spectacle for skywatchers.

While harvest moons happen every year, supermoons occur three to four times a year and partial lunar eclipses happen at least twice yearly. The convergence of all three events, however, is rare.

Here’s what you need to know about this unusual celestial event, and how best to view it.

🌕 What is a harvest supermoon?

A harvest supermoon is a full moon that occurs closest to the autumnal equinox, which is usually around September 22 or 23 each year. It will become immediately visible at moonrise.

As NASA explains, a supermoon happens when a full moon is at or near its closest point to Earth in its orbit. This proximity makes the moon appear about 7% larger and 15% brighter than a typical full moon.

When this full moon also coincides with the equinox, it is referred to as the harvest moon. According to Almanac.com, this special alignment provided extra light for farmers during the harvest season, as it rises close to the horizon for a few evenings in a row.

⏰ When does the partial lunar eclipse start?

According to NASA, tomorrow's partial lunar eclipse will begin at 8:41 PM ET, when it starts entering the partial shadow of Earth. The peak of the eclipse will be at 10:44 p.m. The moon will finish exiting the full shadow at 11:16 PM and the partial shadow on Wednesday morning at 12:47 a.m.

A partial lunar eclipse occurs when only a part of the moon passes through the Earth's umbra, the darkest part of its shadow. During a partial lunar eclipse, the moon's surface becomes partially darkened by Earth's shadow, creating a dramatic visual effect as the moon moves through different stages of partial obscuration.

Unlike a total lunar eclipse, where the moon is entirely covered by the umbra (the first shadow cast during an eclipse), a partial eclipse only affects a portion of the moon's surface.

🔭 How to see the harvest supermoon eclipse

Locate the eastern horizon. To track the moon starting at moonrise, find a spot with a clear view of the eastern horizon. "The lower and less built-up your horizon, the better your chance of seeing the Harvest Moon rising in all of its glory – a fantastic sight that heralds the start of autumn," according to BBC's The Sky at Night.

Check local moonrise times. Apps like My Moon Phase can help determine when the moon will be rising or setting where you are. The NOAA website also has a customizable moon table that provides specific moonrise and moonset times based on your location.

Check the weather forecast. Make sure it's a clear, cloudless night. Websites like Good to Stargaze can tell you whether or not your location's forecast is favorable for seeing the moon.

Prepare your equipment. While the harvest supermoon eclipse will be visible with the naked eye, binoculars or telescopes can help enhance your viewing experience. According to NASA, it's best to choose binoculars with a minimum magnification of 7. Though a magnification of 10 or 15 will provide more detail, you may need a tripod to steady them.

📸 What a harvest supermoon looks like

Here’s a look at what skywatchers might expect to see during tomorrow night’s harvest supermoon.