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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Your Venus assignment . . .

. . . should you accept it, is – observe seven weeks of Venus with a special treat February 27th!

The position of the moon, Venus, and the Sun just after sunset as shown in Stellarium software.

The position of the moon, Venus, and the Sun just after sunset as shown in Stellarium software.

With naked eye:

  • Watch Venus at least once a week for the next seven weeks just after Sunset.
  • Make a drawing showing it’s changing position each week.
  • Check your drawing against the actual positions of Venus and the Earth as they circle our star, the Sun.

Ponder: Why do you see what you see? That is, think about the actual positions of your spacestation (Earth), Venus, and the Sun and relate that to what you see in the sky. This is an excellent opportunity to develop an intuitive sense of the two realities we face – the reality of what we see in our sky and the scientific reality of how ourplamet – and the others are moving about a star.

With binoculars:

  • Do the naked eye observation above
  • Use binoculars to look for any sign that Venus is beginning to show something of a crescent. Or for that matter, any sign of  it going through any phase.  This is a challenge and it is important that you do this as soon after sunset as you can see Venus – then it will be at its highest point in your sky and the background will not provide too much contrast, since it will still be twilight. Draw what you see and note the date. It will change as the weeks wear on.

(Yes, you can see Venus in the day time with binoculars – and sometimes with your naked eye – but I don’t like to encourage looking for objects near the Sun – too much danger of accidentally looking at the Sun and that will damage your eyes – very bad. So wait for the Sun to set – but observe immediately afterwards.)

Ponder: Again, why do you see what you see? That is, think about the actual positions of your spacestation (Earth), Venus, and the Sun and relate that to what you see with your binoculars in terms of the phases of Venus.

With telescope (any size):

Galileo was using about 20X with a terribly small and poor objective lens. Any modern instrument of any size will be better! So you can certainly see what he saw – and should be able to figure out what he figured out, especially since you already know what he believed, but needed to prove – that the planets go around the Sun.

  • Do the naked eye assignment above.
  • With your telescope observe Venus as soon after sunset that it is visible. When I looked for it February 1 it popped into plain, naked-eye view three minutes after Sunset. (Yes, you can see Venus in the day time with a telescope – and sometimes with your naked eye – but I don’t like to encourage looking for objects near the Sun – too much danger of accidentally looking at the Sun and that will damage your eyes – very bad. So wait for the Sun to set – but observe immediately afterwards.)
  • Use the same telescope and same eyepiece each time you observe and note the size of Venus as best you can. (Ideally you would use an eyepiece with a reticle that allowed you to actually measure the size.)
  • Note the phase of Venus and make a quick drawing to indicate it.  Be sure to date your observations.

Ponder: Again, why do you see what you see? That is, think about the actual positions of your spacestation (Earth), Venus, and the Sun and relate that to what you see with your telescope  in terms of the changing size and  phases of Venus.

Resources for naked eye, as well as binocular and telescope observation:

It’s good practice to use the altitude/azimuth system, however.  The first number in the sequence below is the azimuth – that is the compass bearing. In this syetm south is 180, southwest, 225, and west 270.  Altitude is degrees above the horizon. Remember, your fist with arm extended covers roughly 10 degrees.

Here are the azimuth and altitude coordinates for Venus for the Friday of each week during the next two months. These are for my location at nearly 42-degrees North latuitude, though are a reasonable approximation for folks near this latitude. For others, I suggest you consult Stellarium or similar software.

These are the position just after local Sunset.

2/6 233, 38
2/13 241, 37
2/20 248, 35
2/27 256, 31

This last date is special. Venus will appear to be cradled in the arms of a crescent moon, barely 3 days old – or nearly so – what you see will depend on your location, but both the moon and Venus should fit nicely in the same binocular field.

3/6 263, 26
3/13 271, 18

At this point you need a really clear western horizon. By the 20th it will be just one fist – 10 degrees – above that horizon and difficult to spot.

3/20 278, 9

March 20th or 21st is about the last day I expect to easily see Venus during this sequence. I just don’t like playing games with the Sun and it is getting too close and too low.  By the next Friday – the 27th – it sets just 10 minutes after the Sun. Near that date I plan to go to where the ocean is my western horizon.

Actually in the couple days before March 27 you may find it easier to catch it in the morning sky. On the 27 – when it is between us and the Sun – it rises more than half an hour before the Sun in the Northeast at Azimuth 75 – again, be careful and be aware of Sunrise in your location. You don’t want to be looking at Venus in binoculars or a telescope and instead catch the rising Sun. Any glimpse at the Sun through an unfiltered optical instrument can seriously damage your eyes. But you can follow the crescent show in reverse after this if you are an early riser. We see Venus  in both our morning and evening skybecause Venus is north of the Sun. For example, on Macrh 27 the Azimuth of the Sun at Sunrise is 84 degrees, the azimuth of Venus is 75. (Yes – yes – spring is that close! )

Background

Ptolemy

Ptolemy

For Galileo, the view that Venus showed phases like the moon was a good proof that the Copernican theory was correct – the planets all revolved aorund the Sun. Ptolemy was wrong.

I used to accept that as obvious, for it’s pretty easy to see how our view of Venus would cause it to go through phases. However, think about the Ptolemaic system a little with the Earth at the center and suddenly the water gets a bit muddy. I mean, if the moon shows phases while going around the Earth, why wouldn’t Venus? And as I started to think about that it occurred to me that Venus does not APPEAR to go around the Earth. Whoa!

We see it in the morning sky in the east  and we see it in the evening sky in the west – but we never see it move from east to west as every other star does, including our Sun. So it does not appear to go around the Earth – yet the Ptolemaic system said it did.

Make sense? Not to me.  The Ptolemaic system puts the Earth at the center, then the Moon, Mercury, Venus, the Sun, etc.  But we don’t see Venus ( or Mercury) go across our sky. The ancients would not have seen the phases – that needed a telescope – but they certainly knew Venus and Mercury only appeared in the East or the West – never over head. Why?

In trying to understand the answer to that question I learned a lot more about the Ptolemaic system than I ever knew and I gained a much deeper understanding of why the news Galileo brought with his telescope 400 years ago was so disturbing to so many people – they had such a perfect universe where everything made sense and fit into a nice tidy concept.

Well, the concept as Aristotle developed it was nice and tidy. But it didn’t always match observation. It took Ptiolemy to come up with a scheme that would account for Venus and Mercury being only in the eastern or western sky. It was his epicycles that held the key. If you want to understand it, I suggest you read this. You migth also go to this Web page and scroll down to the section on the observations of Venus – there’s a helpful animation there as well.

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