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Today, we are going to take a look at parallax, or more so, we’re going to take a look at the transit of Venus. We’re also going to take another look at the occultation of Venus that’s coming up later this year.

We’re going to jump forward to the same event that we took a look at in the last piece, which is on the 19th of September. You may remember from last time that the Moon and Venus had an occultation, or at least there was an occultation as visible from Ireland during the day. I will go through the occultation just to show it again for anyone who didn’t catch the last piece. Close to 1 o’clock, Venus appears to pass behind the Moon and then pop out the other side. This all happening very much during the day for us here in Ireland, so it is going to be difficult to observe. Of course, with things like these, they are occasionally only visible from certain locations on the Earth, and this occultation seems to be one of those events. It proved a little tricky to catch, because when we moved to a location on Earth where the Sun would still be below the horizon when this occultation occurred, it unfortunately put Venus and the Moon out of alignment. Even from a location where, for the time where this occultation occurs, both Venus and the Moon are above the horizon and the Sun is not, the Moon and Venus do not line up. It looks like the reason for this is the 6,000 kilometer jump across the planet from ireland to Canada. That puts us in a very different location, which means we’re going to see the Moon and Venus from a very different angle. The only location I’ve been able to find where the occultation looks like an occultation and is above the horizon is from here in Ireland, but that’s only visible when the Sun is also already above the horizon.

It’s presumably true for a few other locations where the Sun won’t be quite as high. If we don’t move as far away from Ireland we should be able to see an occultation happening. From the Mid-Atlantic you might notice that Venus is only just barely getting occulted by the Moon. It doesn’t look like Venus is passing directly behind the Moon, more making sort of a chord across the bottom of the Moon. If we jump back to Ireland, it does look like Venus is much more passing through the center of the Moon. Even then, it doesn’t look like it’s perfectly passing through the center of the Moon, it looks like this would be slightly more lined up from a different location. The Moon seeming to be in a different position, compared to the background Venus, from a different location on Earth, is due to an effect known as parallax.

To see parallax with another event, we’re going to keep looking at Venus. Venus doesn’t just get occulted by the Moon, if things line up properly, Venus will get in front of the Sun, giving us a transit of Venus. The most recent wasn’t that long ago, just in 2012, but this does unfortunately mean that the next one will be quite far in the future. We’re going back just a few years and also going back to a slightly earlier point in the year, just a couple of weeks ago, to the 4th and 5th of June. Most of the transit of Venus was not visible from Ireland. If we move through time looking at Venus and the Sun, Venus will get closer to the Sun as the transit begins, and then set for us in Ireland. Venus doesn’t quite pass in front of the Sun from our perspective here in Ireland. Following the Sun and Venus under the ground, it does happen once the Sun is very comfortably under the horizon. We will have to jump to somewhere else to see the transit, but before we do, we’ll keep following the transit of Venus. The transit still hasn’t quite finished when the Sun rises again. As such, there is a chance that the very end of the transit was visible from Ireland. The very end of the transit was visible from other places in Europe, but only the end. The very beginning of the transit was visible in North America, but not the end. The vast majority of the transit was visible across the Pacific, so that’s where we’re going to go. We’re going to head to the Pacific Ocean and a little bit closer to the equator than Ireland. Starting with the End of the transit, and we’re seeing it close to midday. Following the transit all the way back to the start, it begins just around sunrise.

If we move a little bit over to the west coast of North America, this is where you could see the transit beginning. It’s going to start once the Sun gets above the horizon, during the day. The transit begins with edge of the dark circle of Venus touching the bright disc of the Sun. This is the first contact of the transit. When the disc of Venus fully moves onto the Sun, and the other edge of Venus’s disc lines up with the Sun from the inside, that is the second contact. Taking a closer look at that contact is often hard. I’m only seeing it in simulation, which won’t display it perfectly. The edge of Venus seems sharp, but normally a little bit of smudge. It’s very hard to see when exactly Venus is after fully crossing over onto the disk of the Sun, there’s normally a little bit of a distortion. The distortion makes it kind of look like Venus is pulling away of the blackness of space in the same way glue stretches when its getting pulled. That’s known as the black drop effect, and it makes measuring exactly when the transit starts and exactly when the transit finishes a little tricky.

The reason why this matters is because the time when you see the transit starting from the west coast of North America, that’s going to be a little bit different to the time that you see the transit beginning from the middle of the Pacific. If anything this is easier without the black drop effect being simulated properly. It looks like the exact moment of second contact was happening at 3:12, local time. Stellarium shows local times with a correction for the time zone. Stellarium is showing the very accurate correction of -7:12, which is a bit more accurate than time zones normally are. The time zone used on the west coast of North America is Pacific time, usually -7 hours from UTC. If we change our location now to the middle of the Pacific, maintaining focus on Venus and the Sun, the time will look different, however the time adjustment is different as well. Stellarium has actually kept the time being simulated exactly the same, but with this different adjustment due to the different time zone. Most importantly, what we can see here is that Venus looks like it’s in a very different position. It doesn’t look like it’s just crossed over the disk of the Sun yet, it’s still in between first and second contact. The amount of this difference can be measured to help triangulate the distance to Venus and the distance to the Sun, and the distance to the Sun in particular is important. The distance to the Sun when measured like this, when measured using a trigonometry, it’s known as solar parallax.

Looking a the beginning of the eclipse, close to sunrise with the Sun and Venus low in the sky, makes the black drop effect more evident in Stellarium. This probably due to the simulated atmospheric distortion, but it does look like there’s a lot more bleed happening as Venus comes onto the disk of the Sun. It looks a lot more fluid, kind of like Venus is budding off of space. That little bit of blurriness is closer to what the black drop effect actually is, though the simulation is still just a simulation.

Again, we’ll zoom in as close as we can and we’ll get Venus right over to the other edge. We’ll leave the seconds tick forward until Venus has just made, its third contact. This is when Venus’s outer edge first touches the disk of the Sun on its way out. When the other side of Venus passes over all the way to outside the Sun, that will be the fourth contact and the end of the transit. Looking at the third contact at the end of the transit, we know that we were able to catch the very end of the transit from parts of Europe. We’re going to hop all the way around again, but just to make things fair, we’ll keep things pretty close to the equator. We’ll hop all the way over to Africa, and we’ll zoom back in on Venus. With the time remaining the same, Venus is no longer just at third contact. Instead, it is very distinctly inside the disk of the Sun.

By measuring the times of the beginning and end of transit from different locations on Earth, you can get a rough idea of the distance from Venus to the Earth and to the Sun. You’re essentially making a triangle with two points on Earth being the base of the triangle and then Venus being sort of the tip of the triangle. Then you extend that triangle out the other side towards the Sun. The angle at the tip of the triangle, at Venus, has to be the same as the angle of the reflected triangle on the opposite side. This forms two triangles, and then using trigonometry you can figure out the distance to the Sun. We can see some pretty big jumps with the transit of Venus, because it’s so small and because it’s in front of a much bigger thing, it’s a lot easier to see how it’s moving across the Sun. A similar effect of parallax can be seen with eclipses, and as we’ve seen, with occultations.

That’s just a little bit on parallax to hopefully explain why the occultation of the Moon in the last piece was so difficult to see, and to revisit this recent transit of Venus, which was a pretty historical event. We may take a look at some other further in the past transits of Venus and talk a little bit more about parallax and stellar parallax. Stellar parallax is a similar thing, but when we’re looking at the stars, and the two different locations are different parts of the Earth’s orbit, or times of the year. You can look forward to that in a future piece, and if you enjoyed this one then please do like it. If you enjoy this kind of content, please do subscribe to this website and my YouTube channel. Most importantly, thank you for reading and hopefully I’ll see you back here next time.