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.
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.
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.
You can start preparing for an amazing stargazing event: the Perseids meteor shower! The event has already started in mid-July, and can still be observed until the 24th of August. Its peak will occur on August 12, so make sure you organize a stargazing session soon! Thus, if the weather is clear and the nights are dark – make sure you go out somewhere and look up in the night sky, in the direction of the Perseus constellation!
What is a meteor shower?
A meteor shower on Earth usually occurs when our planet’s path intersects with the orbit of a comet. When a comet approaches the Sun, some of its ice vaporizes, leaving behind a stream of dust and debris, called a “dust trail” (which is different from a comet’s tail). When such debris – called meteoroids or micrometeoroids, in function of the size, and which is most of the time the size of a grain of sand -, enters Earth’s atmosphere at very high speeds (typically 70 km/s), it heats up because of the friction with the air in the atmosphere, which causes the particles to light up and glow. This streak of light crossing the night sky is called a meteor, or shooting star. So no, a shooting star is not a real “star” 😉
Meteors usually occur in Earth’s atmosphere at an altitude of above 50 km, and under 100 km. The glow can be fainter and shorter for smaller particles and it becomes brighter and longer as the size of the particle increases. The colour of a meteor can also vary, in function of the chemical composition of the particle!
And, by the way, a meteor that doesn’t burn up and which finally hits Earth’s surface, is called a meteorite!
What is very interesting is the fact that the meteor particles in a meteor shower originate from a point called the radiant, and are all travelling in parallel paths. But if we look at the sky, we see the meteors radiate in all directions. So how can this be? This is the effect of perspective! For example, if you sit in the middle of a straight railroad track and you look along it, you see that the two tracks converge at a single point, somewhere far away. This is exactly what happens with meteors in a meteor shower, but the effect is a lot more intense, due to the great distances where the meteor shower occurs!
Concerning the Perseids now, you should also know that meteor showers are named in function of the constellation where they originate. So, the Perseids seem to originate in the constellation of Perseus, hence their name! The same goes for another well-known meteor shower: the Lyrids, which seem to originate in the constellation Lyra.
Moreover, the Perseids is a predictable event – that is, they occur because of the crossing of Earth’s path with the orbit of the Swift-Tuttle comet, which was last visible from Earth in 1992 (and will next be visible in 2126!). The intersection of Earth with Swift-Tuttle’s orbit occurs each year around July-August, thus, the Perseid meteor shower is then expected!
So, what should you do?
Go outside, away from big cities. Ideally, avoid any source of nearby lighting, including your car’s lights or your phone’s screen. Make, of course, sure that the sky is clear of clouds and try to find the Perseus constellation. To do this, guide yourself with bright stars (with lower magnitudes), such as the Big Dipper asterism and the Cassiopeia constellation: imagine a very thick line between the two and look just below this line, towards the “W”-shaped Cassiopeia. There will be Perseus, and the Perseids will seem to originate from there.
Best is to use your own eyes to see, in order to have a larger field of view, thus no binoculars or telescopes. And make sure you let your eyes adapt to the darkness first! And then comfortably sit somewhere and just look at the sky and let the show begin!
A solar eclipse occurs when the Moon, in it’s own movement, finds itself between the Earth and the Sun. One of the consequences of this alignment will be the blocking of a part (or all) of the Sun’s light here on Earth, in areas where the eclipse is visible. This is due to the fact that the Moon covers the Sun’s disk partially, or sometimes even totally.
Solar eclipses can thus be of two types: partial (when the Moon covers only part of the Sun’s disk) or total (when the Sun’s disk is covered completely by the Moon).
Total Solar eclipse
A total solar eclipse, which is the most dramatic – because the sky turns dark just like at night for a few minutes during mid-day – is much more rare, as the conditions needed for such an eclipse to occur, are much more strict.
For a total eclipse to occur, the Moon needs to be closer to the Earth on its elliptical orbit, so that its apparent size can be large enough to cover the Sun’s disk. In addition, the totality (the period of only a few minutes when the Sun is completely covered by the Moon) occurs only along a very narrow path along the Earth’s surface.
Annular and partial Solar eclipses
So, most of the Solar eclipses are not total!
But even if the Sun’s disk is not covered completely by the Moon, the concentricity of the two disks can create another beautiful Solar eclipse – an annular one, when the Sun is visible around the moon, just like a ring – hence its name!
However, most of the times a Solar eclipse will be partial, when the Sun’s disk will be partially covered by the Moon, and not in a concentrical manner. This is the most common type of Solar eclipse that we see.
Visibility of a Solar eclipse
One solar eclipse (be it partial, annular or total) can’t be visible from everywhere on Earth. The Sun needs already to be visible – which narrows down the visibility of an eclipse to less than half of our planet! As the relative positions of the Moon and Sun in the sky, seen from our planet, are always changing, the alignment of the two for an eclipse to occur, narrow its visibility down even more.
Sizes of the Sun and the Moon
The Sun is much bigger than the Moon, that’s for sure. So how can it be completely covered by the Moon then?
To answer this question, we need to look at the distance of the Sun and Moon, from us, from our planet. The Sun is much more far away from Earth than the Moon is! Which makes it have an apparent size more or less equal to the apparent size of the Moon! The Moon moves along an elliptical orbit around Earth, which makes it, at times, be closer to our planet. Which translates itself into a bigger apparent size of the Moon, which actually becomes slightly larger than the Sun’s apparent size. Which finally, can lead to a total Solar eclipse, if the other conditions for this event to occur, are met!
Looking at an eclipse
Warning! Never look at a solar eclipse with the naked eye. Not even sunglasses aren’t enough! In order to see a solar eclipse, you need a special solar filter, which makes it safe to look directly at the Sun. Otherwise, you risk getting extreme eye damage, and even blindness!
The Annular eclipse of 10 June 2021
On June 10 this year, an annular solar eclipse is scheduled to occur! The actual annular eclipse will be visible from a narrow band along Earth, which crosses the North Pole and a few far-North regions of Canada and Russia.
In the rest of the Northern Hemisphere, the eclipse will be a partial one and will be visible from places such as the Eastern part of the US, almost the whole of Europe and parts of Asia.
If, by any chance, you’ll find yourself in Vadsø, you will see a partial Solar eclipse, with a maximum obscuration of 51%, occuring at 13:09, Norway time.
For other location and times, check out this map from NASA to see exactly where on Earth the eclipse will be visible from, in 2 days!
And by the way, did you know that with Aurora Labs you can discover the mysteries of our own Sun by observing it through our telescope? The eclipse seen through a telescope (with a special solar filter, of course!) is a magnificent sight!
Regions way above the Polar Circle, in the High Arctic, have already started to experience the polar day. Other Arctic regions are going to experience the phenomenon in the coming days. In Vadsø, the polar day already started on May 17 this year, and the Midnight Sun has been visible since, as well.
But what is the polar day?
Earth carries out two types of rotations: one around the Sun, during the course of a year, and the second around its own axis, during 24 hours. At the same time, Earth is inclined in respect with the Sun, at an angle of approximately 23°, and remains tilted at this angle during the whole year.
This means that the Earth is illuminated by the Sun differently during one year. At and around the summer solstice (sometime around 22nd June each year), Earth is inclined in such a way that the North Pole and the region around the North Pole, points towards the Sun, thus it is illuminated more and longer.
Just take a look at the first part of this video from the California Academy of Sciences, and see how Earth is illuminated by the Sun during a whole year.
You can see that the length of the polar day varies in function of latitude: closer you are to the North Pole, longer the polar day is. At the exact location of the North Pole, the polar day lasts no less than 6 months! At lower latitudes, but still above the Arctic Circle, the Sun never sets for a shorter period. The shortest polar day occurs on regions exactly on the Polar Circle (at 66°N), where the Sun never sets for only 1 day, which is exactly the day of the Summer Solstice!
The Midnight Sun
The Midnight Sun is a wonderphul phenomenon. It is what makes the sky bright at “night time” during the polar day, just like the Northern Lights brighten the sky during the dark period. It is a typical Arctic (and Antarctic) phenomenon, which occurs only during the polar day.
As the name suggests, here in the Arctic, the sun is visible in the sky at midnight, as well as the whole night and day, and it never sets below the horizon during this period. In Vadsø, the Midnight Sun will be visible this year until July 26.
And did you know that Aurora Labs has designed a special activity dedicated to discovering the midnight sun differently? Check it out here!
Regions below the polar circle experience a normal day/night cycle, which varies also in length, in function of the exact latitude.
And, by the way, the opposite of the polar day is the magnificent polar night! Have you ever experienced one or the other?
You will often hear the term “magnitude” in Astronomy. Have you ever wondered what it meant? In this article, we’ll try explaining this term and we’ll see how to use correctly the “apparent magnitude” or “absolute magnitude” when talking about astronomic objects in the sky.
What is magnitude?
To keep things simple, in astronomy, “magnitude” refers to the brightness of an object in the sky. What we need to be particularly careful about, is the fact that the brighter the object, the smaller its magnitude! For example: a star with magnitude 1 is brighter than a star with magnitude 2! …And you guessed it, magnitude is unitless, that’s why we say “magnitude 2” or “2 magnitude”.
Apparent and absolute magnitude
Let’s imagine that we are on the top of a hill and we look at a very distant street light, down in the valley; let’s say this street light is 5 km away. From the top of the hill, we can see that the light is of a certain brightness. Now, imagine we start walking towards the street light. As we approach it, it seems that it gets brighter and brighter. So, how can we quantify the brightness of the street light if it seems to vary in function of where we are, relative to it?
In Astronomy, this issue is addressed by using two types of brightness – or, more correctly, two types of magnitude – for a celestial object: its apparent magnitude and its absolute magnitude. Most of us – at least in the near future! – will probably see the Moon, the stars, the planets and any other bright object in the night sky from our own planet, from Earth. All these objects will have a certain brightness, as they are seen from Earth, and this brightness is characterized by the apparent magnitude. So, the apparent magnitude of an astronomical object is the brightness of that object as seen from Earth.
As for the absolute magnitude, it is defined as the apparent magnitude of an astronomical object, as seen from a distance of approximately 310.000.000.000.000 km (the equivalent of 10 parsecs). The “usual” astronomer will just stick to the apparent magnitude; however, the absolute magnitude is important in research and studies, for example, for comparing the “real” luminosities of two or more objects.
Also, when talking about just “magnitude” – thus without specifying “apparent” or “absolute” – it’s the apparent magnitude which we refer to.
Remember that a lower magnitude means a brighter object. But brighter of how much exactly?
The magnitude scale is logarithmic. Which means that the values which are to be displayed and compared on this scale are very far apart: the largest numbers are very much larger than the smallest numbers to be compared. To get a sense of it, magnitude 1 is 100 times brighter than a magnitude 6 (and not just 6 times brighter, as it would be the case on a “normal” scale).
Here are a few examples of magnitudes, to get an idea how this works:
The Sun has a magnitude of -27
The full Moon has a magnitude of -13
The International Space Station, when brightest, has a magnitude of -6
Planet Venus, when visible and when brightest, has a magnitude of -5
Sirius, the brightest star in the sky, has a magnitude of -1
Vega, the brightest star located in the Lyra constellation, has a magnitude of 0
The human eye, unaided, can normally see up to magnitudes of +3 – +6 (in function of the light pollution)
Can you now imagine how much brighter is the Sun (of magnitude -27), which you can’t even look directly at, compared to a Full moon (of magnitude -13)?
Magnitudes can be negative or positive, and the same rule applies: lower the magnitude – brighter the object.
The star Vega, besides being one of the brightest stars in the night sky and besides guiding us to find the Lyrids meteor shower each year in April, is also the reference point on the magnitude scale, having a value of 0.