Oh my goodness! The sky moves? (Lesson 3)

We did a little exercise with some teachers and staff in the Philippines. After teaching them a few basics, it was time to get their heads out of the classroom. The assigned task was simple: from a location outside, do a quick drawing of the boundary between earth and sky in a certain direction (about a 90 degree swath in one compass direction). They had 10 minutes. What they delivered to us to review was dominated by detail on the earth. It said a lot, because their heads were still earthbound. Definitions of the horizon, as defined by where the sky began (going up) or the earth began (going down) were less dominant. Any definition of the sky was barely noticeable. So, we talked about how a mindset change needs to take place to begin to appreciate the created heavens. Their drawings were evidence from their observations that it was necessary. This lesson will help that change in mindset.

So let’s do a three-part exercise in the next couple nights. Get dark adapted; have your little red light ready; take a piece of paper and clipboard;  have your spot to look up already in mind where you can look generally to the South. Go out a little after sunset (there will still be some light in the western sky). Right now the moon is high … just forward of zenith and generally southward if you are in northern lattitudes.

Step 1. Do the same exercise we did in the Philippines. Define the boundary between earth and sky. So you will draw an outline of a boundary where you can no longer see the sky. Pick your spot. Face south. Start by drawing a line straight across the bottom of the paper for an ideal horizon. Above that you will draw things that block the sky (trees, buildings, hills, etc.). Do this for a little over 4 handwidths (measured horizontally), which equates to about 80 degrees.

Step 2. You will see (during the next couple days) about a half-moon showing, which is called a first-quarter moon because we are about 1/4 of the way through the moon cycle. In the vicinity of the moon (within the next couple days), find the nearest brightest “star” that has a reddish orange color. It is actually Mars. Measure the distance between it and the moon with an outstretched hand. Also measure the distance (angle) between Mars and the horizon. Note the position of Mars over something you can easily recognize on the horizon. Make a brief sketch with the results so you remember the angles you measured between the two sky objects and the one from Mars to the horizon.

Note: an outstretched hand and arm means that your eyes are seeing an angle of about 20 degrees from the tip of your thumb to the tip of your little finger. A fist, on the other hand (with the thumb out of sight), is about 8 degrees.

Step 3. Wait at least one and a half hours. Go to the same spot. Note the position of  Mars above the horizon. If you could draw a line straight down to the horizon, where is that point compared to where it was at least an hour and a half earlier? If you have observed carefully from the middle latitudes, Mars will have moved westward about 20 degrees. The distance between the moon and Mars (you can check this) will have stayed about the same. So, the sky “moves” a little more than an outstretched hand every 90 minutes or so. Or does it?

One of the first amazing things to discover and then enjoy rediscovering (and using) is the magnificent clock that the Lord has created. Our rotation means that the sky appears to move. At mid-latitudes (north or south), our rotation becomes abundantly obvious as heavenly bodies march westward at 15 degrees and hour (360 degrees of rotation divided by 24 hours). The axis of that rotation, of course, points (in the north) to Polaris, or the North Star. It is easy to calculate this sky movement but it takes a little longer to get a sense of it for a new observer under the night sky. Think about it. Measure some other objects. Watch a star near the western horizon disappear. Watch and measure stars in the low eastern sky move higher. Then try extending the time. If there are four hours between observations, an object fairly high in the sky will move about 60 degrees westward if you live in the middle lattitudes to the equator.

Look at it a different way. God says in Genesis 1:14 that He made the heavenly bodies (in part) to mark seasons, days, and years. You have just gone the first step to see how things change in a few hours or part of a day. It is a clock more reliable than a watch. There are lots more details that become obvious, but the simple but big first step is to get a sense that the sky “moves” during the course of a night (or day). He made our rotation so we get a good view of a lot of the celestial sphere just by choosing when or for how long we look at the sky.

Enough for tonight. Try the exercise if you have opportunity during the next couple nights. If you observe two different nights, also note how the angle between Mars and Moon change on successive nights. We will talk about  two types of time on the next blog post. Then you will understand why this angle changes. You can do the same exercise but use the moon and any other bright star (or “wandering star” or planet) that you can easily locate.

For now, here is an observation of an open cluster that is high in the sky at mid-northern latitudes in the spring. In June and with the naked eyes, it is a whitish patch just to the west of zenith. You can search on its name by using the terms in the observing sheet. It has the typical elements we teach folks to record when they observe. The sketch is inverted, which means white became black and black became white compared to the original pencil sketch. This is at a magnification of 16 power. The eyes (not using a binocular) will see a whitish patch in a larger field of brighter stars.

…til next time.


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