Event date: March 20th
Time: 6:05 (PM)
Brief
There will be several times that I will use the term astronomical unit (a.u.) to indicate the distance of planets from the Sun and each other. An a.u. is the average, or mean distance between the Earth and Sun. To find this distance, which is approximately 93 million (mi)les, the distances of the Earth from the Sun at perihelion in January, and aphelion in July are averaged. The distance between the two bodies are 91,402,640 mi at perihelion and 94,509,460 mi at aphelion. The table included for the mean distance link above has these units in kilometers (km) instead of miles, so use whichever system suits you best. As a result of ellipse-shaped orbits with the Sun not centered, the other planets of our solar systems have mean distances to calculate also; they can be found using that same link by scrolling up and down from one table to the next on the website.
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Detailed
We will use Venus as example for talking planet-Sun distance, since it reaches perihelion today at the time listed of 06:05 PM, PDT. Venus at this moment, is about 66,784,500 million mi from the Sun. If you have noticed a very bright "star" high in the west just after Sunset over the last several weeks, that is Venus! Reaching perihelion hardly effects its apparent magnitude, which is how bright we see it from Earth. Instead, as Venus comes closer to us every day during this time of its cycle, we are seeing more lit area of the planet. The sulfuric acid clouds covering Venus' entire surface reflect alot of Sunlight, while the difference of Venus in distance and therefore angular size, changes dramatically during the periods that we see it on either side of the Sun.
The first image shows Venus where it is in orbit, zoomed out, while the second shows it more zoomed in.
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When looking at image 1, the dim/thin part of the orbit indicates the curve further from our line of sight, while the bright/thick part is where the planet is when closer to us. Right now, Venus is only about a week away from what is known as greatest elongation, when it will reach its greatest angular separation from the Sun. It is important to remember, that this should not be confused with greatest distance from the Sun, which is its aphelion, mentioned earlier. Being just short of 46º in separation, that is about 1 1/2º from maximum separation from the Sun close to, or at aphelion. In short, when at perihelion, the angular separation is not much different than when at aphelion. However, because Venus is currently further north on our celestial dome than our Sun, we still get to see the planet for almost 4 hours after the Sun each evening, for the rest of the month. The celestial latitude difference between the two, which I defined in yesterday's entry as declination, reached its peak last week, and now are 19º different. When Venus has a greatest elongation east of the Sun during the weeks of between mid-February and early March for the northern hemisphere, this difference makes sense; just as we notice our Sun rising north and setting later at faster rates than other times of year. How does that relate? Yesterday I introduced the ecliptic and celestial equator for the equinox. Now that the Sun is past their intersection, watch the path it makes on the ecliptic over the next few weeks, relative to the celestial equator; Venus has moved along this section of ecliptic already, east of the Sun.
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