Friday, February 3, 2012

Sunspots demonstrate Earth's rotation

How to observe and photograph sunspots
Sunspots are regions on the sun on which the surface temperature is slightly lower. These regions emit less visible energy and from earth they look as dark spots. When looking at the sun with proper eye protection (glasses which are suitable for solar eclipse) the spots are clearly seen.
Warning: Looking directly at the sun, or photographing it without protection is dangerous and can cause permanent eye damage.

This article focuses on how to photograph sunspots. I've used a regular camera with 35X zoom and a special sun-blocking filter which blocks 99.99% of the sun rays (and also blocks UV and IR radiation). Photographing sunspots requires using a dedicated filter, it is not possible to improvise and use for example an exposed films for this purpose!
Sunspot AR1429 6-Mar-2013 morning and evening
Sunspot AR1429 6-Mar-2013 morning and evening


The spots can be seen easily. Looking through a telescope (again, a proper and dedicated filter is a must, using a telescope focuses the sun's energy into a single spot and using a non-proper filter will lead to immediate and permanent damage to your eye) will show even finer details, while looking through a special solar telescope will show details on the sun limb as well.

sunspots
Sunspot

sunspots
Sunspot
Both pictures show the same sunspots (sunspots are officially numbered and these two are 1203 and 1204 in case you wonder). The first photo was taken in the morning and the second just before sunset, and you can see that the location of the sunspots is very different. Why is this? It is not due to the daily movement of the sun (actually it is the earth which moves). It is also not due to the rotation of the sun around its axis (the sun rotates around its axis, and this rotation changes the location of the spots, however, the change is visible after a day or two and not just after several hours). The reason is simply the rotation of earth around its axis. To further explain this, look below at the same photos as above, only now I have drawn in the sun's axis. You will see that the spots are in the same place, relative to the axis, and what has changed is the angle at which we, on earth, observe the sun.

sunspots
Same sunspots in Sunrise (right) and Sunset (left)

To summarize the issue, please watch the following video.



1 comment:

  1. I had trouble visualizing this, because the description makes it sound like the axis of the sun tilts relative to the axis of the earth. This is not what happens.

    I understood it when I pictured myself standing on the surface of the earth at the equator, watching the moon rise directly to the east, and pretending that there's no such thing as axial tilt. If I had the patience to stand there for 6 hours, the moon would be directly overhead. My body would still be facing east, and my neck would be pretty sore from being bent at a right angle.

    After another 3 hours, the moon would be _behind_ me, and I would have to either rotate my body, or bend over backward, if I wanted to see it. So the apparent rotation of the moon's (or the sun's) axis comes not from the moon/sun moving relative to the earth, but from the human turning to face the moon/sun as it moves across the sky. Of course the axis appears to twist as the human rotates their body (and head) through nearly 180 degrees, so that they are always facing the moon/sun as it crosses the sky!

    This also explains why, despite the moon's axis and the earth's axis being almost in the same plane, we often see the moon "lying on its back" with the shadow line apparently horizontal as it rises or sets. If we were standing facing the equator with our shoulders parallel to it, we would see that the moon was upright - but we would have to look over our shoulder to see it at all.

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