What is a blue moon?

what is a blue moon jennifer gray weather orig_00001320.jpg

What is a blue moon?

July 31st, 2015 marked a rare blue moon. CNN's Jennifer Gray explains the phenomenon, and why it only happens once in a blue moon.

01:01 - Source: CNN

Amazing Outer Space 16 videos

what is a blue moon jennifer gray weather orig_00001320.jpg

What is a blue moon?

01:01

Now playing

- Source: CNN

NASA releases first ever ISS video shot at 8K

NASA shoots first 8K video of Earth

01:16

Now playing

- Source: CNN

NASA IOP SS Water Test 3

Why did NASA launch 450,000 gallons of water?

00:59

Now playing

- Source: CNN

In this photo released by NASA, the Northrop Grumman Antares rocket, with Cygnus resupply spacecraft onboard, launches from Pad-0A, Saturday, Nov. 17, 2018 at NASA's Wallops Flight Facility in Va. (Joel Kowsky/NASA via AP)

Watch NASA launch cargo ship into space

00:34

Now playing

- Source: HLN

SANTA BARBARA, CA - OCTOBER 07: The SpaceX Falcon 9 rocket (R) separates from the space craft (L) behind the rocket trail after launching from Vandenberg Air Force Base carrying the SAOCOM 1A and ITASAT 1 satellites, as seen on October 7, 2018 near Santa Barbara, California. After launching the satellites, the Falcon 9 rocket successfully returned to land on solid ground near the launch site rather than at sea. The satellites will become part of a six-satellite constellation that will work in tandem with an Italian constellation known as COSMO-SkyMed. (Photo by David McNew/Getty Images)

SpaceX rocket launch lights up evening sky

01:09

Now playing

- Source: CNN Business

JAXA astroid ryugu images

Robot sends new images from asteroid's surface

00:33

Now playing

- Source: CNN

Massive structure on Saturn

'Bizarre' hexagon topping Saturn's north pole

01:05

Now playing

- Source: CNN

The SpaceX launch of a Falcon 9 rocket marks another milestone for Kennedy Space Center's Launch Complex 39-A.

60 years of NASA's history in 120 seconds

02:00

Now playing

- Source: CNN

nasa parker solar probe

Watch NASA launch probe that will explore sun

01:39

Now playing

- Source: CNN

nasa lagoon nebula new images lon orig_00000000.jpg

Stunning virtual tour of the Lagoon Nebula

01:19

Now playing

- Source: CNN

Artist's concept of the Solar Probe Plus spacecraft approaching the sun. In order to unlock the mysteries of the corona, but also to protect a society that is increasingly dependent on technology from the threats of space weather, we will send Solar Probe Plus to touch the sun.

Listen: The sun is not silent

00:59

Now playing

- Source: CNN

moon 2

Breathtaking virtual tour of the Moon in 4K

01:26

Now playing

- Source: CNN

This composite image, derived from data collected by the Jovian Infrared Auroral Mapper (JIRAM) instrument aboard NASA's Juno mission to Jupiter, shows the central cyclone at the planet's north pole and the eight cyclones that encircle it.

NASA mission discovers Jupiter's inner secrets

01:02

Now playing

- Source: CNN

earliest light

Scientists detect 'fingerprint' of earliest light in the universe

00:46

Now playing

- Source: CNN

super blue blood moon

'Super blue blood moon' lights up the skies

00:46

Now playing

- Source: CNN

what is a black moon orig jpm_00000000.jpg

What's in a moon's name?

01:04

Now playing

- Source: CNN