When you look at the Moon, sometimes, besides the illumination of the crescent of the moon, you are able to see the rest of the disk of our planet’s natural satellite. That dim illumination of the darker disk is called earthshine.
What causes it?
But what causes this illumination? We all know that sunlight hits Earth, there’s no question there. But Earth is a globe, and a fraction of the light that hits our planet, is reflected back into space (and in the scientific literature is known as earthlight). Some of this light that gets reflected into space, hits the moon as well, and it is this what causes the lighting up of the dark disk besides the crescent moon. And this dim light gets reflected back to Earth and it’s what our eyes percieve.
Sunlight hits Earth (yellow arrow). A portion of this light gets reflected back into space (white arrow) and can cause earthshine, which is reflected back to Earth (grey arrow).
When to look?
When the moon is full, we’ve all seen that it can light up pretty well entire regions on our planet, from where the full moon is visible. From the surface of the moon, Earth can be also illuminated by the Sun (just like other planets of our Solar System). When Earth is “full” (from the moon’s night sky perspective), it means that our planet can light up an entire region on the moon as well. Why would it only be the other way around, anyway? This also means that earthshine is visible at its maximum when Earth is almost “full”, so, when the Moon is a very thin crescent.
Planetshine
This phenomenon occurs on other celestial bodies as well, not only on Earth! When a planet is illuminated by its star, and if that planet has a natural satellite, the phenomenon takes plase in the same waym just like in the Sun-Earth-Moon system. The only difference is the name – it’s generally called planetshine. And, of course, light reflected from a planet in general, is called planetlight.
Natural satellites are natural celestial bodies that orbit around a planet of the Solar System. But did you know that another commonly used term for denoting a natural satellite is “moon”? You’ve all probably seen the Moon in the night sky! The Moon is Earth’s natural satellite that continuously orbits around our planet. However, other planets of our Solar System do have natural satellites, or moons!
Jupiter’s Moons
Probably the best known moons (other than our own planet’s Moon) are Jupiter’s: Ganymede, Callisto, Io and Europa. Jupiter has 79 moons in total, but these 4 are the biggest and the most visible from Earth! They’re called Galilean moons, because they were first discovered by Galileo Galilei in 1610 as the first natural satellites that would orbit another planet than Earth! The 4 Galilean moons are visible in the night sky with almost any telescope and are a beautiful sight during a stargazing session.
Animation of the Galilean Moons. Image credit: NASA.
Saturn’s Moons
Saturn has got 82 moons. However, as the planet lies further away from Earth than Jupiter, and because the majority of the ringed planet’s moons are small in diameter, only one – Titan – is rather easily visible through a telescope from Earth.
Saturn moon transit. Image credit: NASA.
Other Moons
Mars, Uranus and Neptune also have their own moons, all varying in sizes from about 10 km diameter to as big as Neptune’s moon Triton, of approximately 2710 km in diameter!
The Moon
But now, coming back to earth’s Moon – did you know that this natural satellite is the brightest celestial body visible in the night sky? Its apparent magnitude varies from -2.50 during the New Moon phase, to -12.90 when it’s a Full Moon.
By the way, apparent magnitude measures the brightness of a celestial body (star, moon, satellite or any other astronomical object) observed from Earth. The lower the magnitude is, the brighter the object appears. As information, the Sun in the daytime sky has the apparent magnitude of -26.74 and the unaided human eye can see in the night sky magnitudes of up to around +3 (in relatively high light-polluted areas) or +5 (under very clear dark sky conditions).
Full Moon
…And did you also know that the Moon’s apparent size in the sky is almost the same as that of the Sun? The Moon’s diameter is, of course much, much smaller than that of the Sun: approximately 3500 km vs. the Sun’s 1.4 million km. But the relatively short distance between our planet and the Moon (384400 km), compared to the distance from Earth to the Sun (150 million km), makes the apparent size of the two celestial bodies approximately the same. Moreover, this relatively similar apparent size makes it possible for the Moon to cover the Sun almost precisely during a total solar eclipse! However, as the Moon’s distance from Earth continually increases, this “perfect” match between the two similar apparent sizes will stop occurring in the far future and total solar eclipses will not be possible anymore.
Oh, and did you know that the Moon is also responsible for the tides? Ocean tides are the continuous rise and fall of sea levels, which occur because of the concurrence between the Moon’s and Sun’s gravities, combined with Earth’s rotation! But more about this subject in another article!
The polar day has almost arrived in Vadsø and the night sky is almost as bright as during the daytime. Therefore, the stars, planets and other sky objects will not be visible anymore during “classical” stargazing sessions, under a dark night sky! But we at Aurora Labs, did find a possibility to enjoy the sky from an astronomical point of view nevertheless – even in daylight!
The International Space Station, or ISS as it is commonly known, is today the largest man-made object that flies in space, at around 400 km altitude above Earth. And because it has the size of a football field, it is big enough to be seen even from our planet!
The International Space Station orbiting Earth
Things are easier at night: if you look at the sky, the ISS will appear as a very bright star (approximately the same brightness as planet Venus). Because it is so bright, it should be seen even from urban areas! The bright dot suddenly appears on the horizon, moves steadily without changing speed or direction, and disappears again below the horizon. And it doesn’t blink as a plane does. If you’ve ever seen something like this, chances are that you’ve spotted the ISS! In addition, there are online tools and phone apps which calculate for you when ISS should be visible in your exact location – such as, for example, Spot the Station, provided by NASA.
You don’t need a telescope to see this show; however, to the naked eye, the ISS looks like a bright dot, with no other features. A telescope would permit you to see some features, as solar panels for example.
During the day, things are more difficult. At night, ISS is seen because it reflects the light of the already set Sun. During the day, however, a possibility to see ISS is when it passes across (or transits) the face of the Sun. And what a great sight that is! Be careful though, in order to see this great show, you need a properly equipped telescope with solar filters when you look at the Sun, otherwise the light of our star is so bright, that it can create even blindness!
Multiple photographers around the world have taken such photos of the ISS transiting the Sun or moon. ISS travels at a speed of around 28.000 km/h, so, can you imagine how precise you need to be in order to capture such a moment that only lasts less than half a second? Because – yes – the ISS transiting the Sun or moon lasts less than 0.5 seconds!
One of the most recent photos of this kind, is the one taken by the photographer Mack Murdoc from Los Angeles, which is a composite photo showing the ISS as it passed across the Sun.
During daytime hours, besides the ISS, it is possible to see even planets, such as Venus, transiting the Sun! In the photo below, the black dot is not a sunspot, as you might believe, but it’s no other than planet Venus!
Gestrgangleri (https://commons.wikimedia.org/wiki/File:20040608_Venus_Transit.JPG), „20040608 Venus Transit“, marked as public domain, more details on Wikimedia Commons: https://commons.wikimedia.org/wiki/Template:PD-self
And here, meet Mercury as the small black round dot (you can also see sunspots in this picture, and how different they look compared to the perfect round shape of a planet):
Elijah Mathews (https://commons.wikimedia.org/wiki/File:Transit_Of_Mercury,_May_9th,_2016.png), „Transit Of Mercury, May 9th, 2016“, https://creativecommons.org/licenses/by-sa/4.0/legalcode
The Moon is also sometimes visible in the sky during the daylight, and with a properly equipped telescope, you can see its features really well, too!
The Moon seen during the day
Last but not least, our Sun is a star, just like the million others out there that you can see in the night sky! Therefore it can be observed with a telescope as well! A specially equipped telescope with solar filters, of course, to protect your eyes from the extremely bright light! If you visit me here in Vadsø, I offer this activity as part of the Cloud Spotting under the Midnight Sun activity and we’ll have a look at our star and at its sunspots!
