I was quite determined that I shouldn't let another year pass from my last post. The trip last week which was a failed Orionids attempt gave me an opportunity capture a well understood, but less known phenomenon relating to the Moon. Given the International Observe the Moon Day already a day behind (28th of Oct), I figured I'll make use of those.
When we look at the Moon, we see the sunlight which is reflected by the Moon. When it is the full moon, the Moon is on the opposite side of the Sun when seeing from Earth. Therefore the whole 'face' of the Moon which is facing the sun can be seen. When it is the new moon, the Moon is between the Sun and the Earth and therefore we cannot see the illuminated side of the Moon.
There is something interesting that happens when the Moon starts moving from the new moon phase to the full moon phase. Right after new moon, when the Moon is at crescenet level, you can still see the whole of the Moon at a lesser brightness. This phenomenon is called earthshine.
In the photo above (taken around 5-6 days after the new moon), the brighter side is the reflection of sun light. We can see only part of the Moon because we are at an angle 'behind' the Moon.
The diagram above shows that while the Moon has half of its illuminated all the time, due to its position, how an observer from earth sees only either part of it (during crescent) or none of it (during new moon). This raises the question as to how we see the supposedly darker part of the Moon at a lower brightness as shown in the photo above.
This phenomenon is called earthshine. So how does the Moon really illuminate? This is because the sunlight is also reflected from Earth and falls on the Moon. This light is reflected light is again reflected off the face of the Moon and reflected back to Earth. This also explains why that part of the Moon is dimmer. It is the sunlight reflected twice, once from the face of the Earth and then from the face of the Moon.
The measure of reflection of sunlight is called the albedo. Albedo is a number that ranges between 0 - 1. A perfect blackbody will have an albedo of 0 where as a perfect reflecting body will have an albedo of 1. Earth on average has an albedo of 0.3 meaning that about 30% of the sunlight fallen on earth will be reflected. Moon has an albedo of around 0.12. Thus while 30% of the sunlight is reflected off of Earth, only 12% of the sunlight will be reflect off of the Moon.
Can we then see earthshine throughout the phases of the Moon although we don't pay attention to it? Not really. Once the Moon passes the half moon phase, the Moon moves behind the illuminated face of the Earth. Which means almost no reflected light from Earth falls on the Moon.
Even as you get to the half moon phase, the reflected sunlight from the earth loses its intensity due to the light being scattered. Therefore once past half moon phase no earthshine occurs. Above diagram also explains why earthshine is brightest within a few days of the new moon, that's when the intensity of the reflected light is at its highest.
One last question to answer, does the earthshine occur on new moon as well? I.e. when you are not supposed to see the Moon at all, would you still see it because of earthshine? According to NASA, yes you can: https://earthobservatory.nasa.gov/IOTD/view.php?id=83782
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| Moon - 21 Oct 2017 |
When we look at the Moon, we see the sunlight which is reflected by the Moon. When it is the full moon, the Moon is on the opposite side of the Sun when seeing from Earth. Therefore the whole 'face' of the Moon which is facing the sun can be seen. When it is the new moon, the Moon is between the Sun and the Earth and therefore we cannot see the illuminated side of the Moon.
There is something interesting that happens when the Moon starts moving from the new moon phase to the full moon phase. Right after new moon, when the Moon is at crescenet level, you can still see the whole of the Moon at a lesser brightness. This phenomenon is called earthshine.
In the photo above (taken around 5-6 days after the new moon), the brighter side is the reflection of sun light. We can see only part of the Moon because we are at an angle 'behind' the Moon.
The diagram above shows that while the Moon has half of its illuminated all the time, due to its position, how an observer from earth sees only either part of it (during crescent) or none of it (during new moon). This raises the question as to how we see the supposedly darker part of the Moon at a lower brightness as shown in the photo above.
This phenomenon is called earthshine. So how does the Moon really illuminate? This is because the sunlight is also reflected from Earth and falls on the Moon. This light is reflected light is again reflected off the face of the Moon and reflected back to Earth. This also explains why that part of the Moon is dimmer. It is the sunlight reflected twice, once from the face of the Earth and then from the face of the Moon.
The measure of reflection of sunlight is called the albedo. Albedo is a number that ranges between 0 - 1. A perfect blackbody will have an albedo of 0 where as a perfect reflecting body will have an albedo of 1. Earth on average has an albedo of 0.3 meaning that about 30% of the sunlight fallen on earth will be reflected. Moon has an albedo of around 0.12. Thus while 30% of the sunlight is reflected off of Earth, only 12% of the sunlight will be reflect off of the Moon.
Can we then see earthshine throughout the phases of the Moon although we don't pay attention to it? Not really. Once the Moon passes the half moon phase, the Moon moves behind the illuminated face of the Earth. Which means almost no reflected light from Earth falls on the Moon.
One last question to answer, does the earthshine occur on new moon as well? I.e. when you are not supposed to see the Moon at all, would you still see it because of earthshine? According to NASA, yes you can: https://earthobservatory.nasa.gov/IOTD/view.php?id=83782
A new Moon occurs when all of the Sun’s light is reflected away from Earth, and the side of the Moon facing Earth is barely visible, as illustrated in the above figures. Sometimes the dark face of the Moon catches Earth’s reflected glow and returns that light. The dark face of the Moon has a faint shine, a ghostly version of a full Moon.And now I have another item added to my ToDos. To catch the Moon on a new moon day.
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