Event Date: April 14th
Time: 8:00 PM
Brief
Venus is approaching an important date during its post-greatest eastern elongation (g.e.e.) time in our evening sky. Although it is gradually coming back towards the Sun and its apparition will become increasingly worse over the next month and a half, it is becoming a more attractive target to look at in a telescope: it is waning during its crescent phase, and increasing in angular size as it does. While it does so, it is also catching up with us in orbit, and therefore becoming slightly brighter for the next two weeks. Since it emerged from the Sun last fall, it has brightened by a larger percentage, from the high -3 range to mid -4 on the apparent magnitude scale. As for the important date, Venus is approaching greatest brilliancy. This is when we see the greatest surface area of light reflected to our eyes. It doesn't mean that Venus is at its brightest however, as mentioned in the detailed section. Before getting to that, here is Venus shown in its orbit, approaching Earth in our own. Notice how it is now on the thicker, more visible part of the orbit, as we view its orbital path at an angle. This evening, Venus is approximately 53 million miles from us, after being as far as 93 million miles from us (1 a.u.) as recently as February 16th.
Time: 8:00 PM
Brief
Venus is approaching an important date during its post-greatest eastern elongation (g.e.e.) time in our evening sky. Although it is gradually coming back towards the Sun and its apparition will become increasingly worse over the next month and a half, it is becoming a more attractive target to look at in a telescope: it is waning during its crescent phase, and increasing in angular size as it does. While it does so, it is also catching up with us in orbit, and therefore becoming slightly brighter for the next two weeks. Since it emerged from the Sun last fall, it has brightened by a larger percentage, from the high -3 range to mid -4 on the apparent magnitude scale. As for the important date, Venus is approaching greatest brilliancy. This is when we see the greatest surface area of light reflected to our eyes. It doesn't mean that Venus is at its brightest however, as mentioned in the detailed section. Before getting to that, here is Venus shown in its orbit, approaching Earth in our own. Notice how it is now on the thicker, more visible part of the orbit, as we view its orbital path at an angle. This evening, Venus is approximately 53 million miles from us, after being as far as 93 million miles from us (1 a.u.) as recently as February 16th.
I will borrow some interesting text from the wikipedia link, for anyone confusing greatest brilliancy with greatest brightness. Admittedly, I was one of them, until I read this!
http://en.wikipedia.org/wiki/Aspects_of_Venus
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Greatest brilliancy is often confused with "maximum brightness"; although they are related, they are not quite the same thing. Greatest brilliancy is really a geometric maximum: it occurs when the apparent area of the sunlit part of Venus that we see from Earth is greatest. Only if the luminance of Venus' apparent surface would be constant (i.e. the same at every point and at every phase) would the greatest brilliancy of Venus coincide with its maximum brightness. However, the reflection of sunlight on Venus more closely follows Lambert's law, which means that the maximum brightness occurs at a somewhat larger phase of Venus than its greatest brilliancy.
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The two numbers to look for in calculating Venus' surface area, are (1) its percent illuminated, and (2) its angular size. Once its angular size is researched in arcseconds, as software or the internet can also reveal, the formula for calculating the area is the same as that for any other circle: pi (π) multiplied by the radius-squared. To keep it simple, we will keep pi at 3.14, rather than several decimal places extra. Since the angular size is measured in diameter however, divide that by 2 first, and then apply the formula. This evening, with Venus at 30 arc-seconds in diameter, the radius is 15. The area therefore, is 3.14*15*15= 706.5 square arc-seconds. Now, with the planet 39% illuminated this evening, multiply this in decimal form with 706.5, which equals 276 area arc-seconds , rounded up.
note: since angular measurements are not square, I redundantly reuse the word 'area' in the previous sentence.
Until about 6 weeks before inferior conjunction, this area reaches its greatest value and therefore, greatest brilliancy. As the borrowed text above states however, this often falls dates before or after the greatest magnitude that we see Venus. With the eye alone, we would not notice, and the overall value in magnitude is still very close.
This second image shows Venus zoomed in, showing the 276 sq. arcs-econd surface area. Use the numbers above again as a reminder, to understand the image, using a field of view of 8 arc-minutes.
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| *click on images to enlarge: courtesy of Starry Night Pro Plus, version 6.4.3, by Simulation Curriculum Corp. |
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