# How much dimmer is a partial solar eclipse that goes from -26.71 to -25.89?

Based on what I found in Stellarium, for Vlissingen, Netherlands, on July 2, 1666 there was a partial eclipse with a change of apparent brightness -26.71 to -25.89.

Is it valid to say based on the chart below that the sun would've appeared a little more than 50% dimmer at maximum compared to before the eclipse?

If not, approximately how much dimmer was the sun during this eclipse, based on the values given by Stellarium?

Source

above: Just before the start of the eclipse. below: At maximum

• +1 I find it much easier to read the words first, so I've moved the images to the bottom instead of having to look around them mid-sentence. Please feel free to "roll back" if you prefer the original flow. – uhoh Mar 7 at 7:31
• If you are asking whether the people would notice the eclipse, in a partial solar elicpse in southeastern Pennsylvania I noticed that under the trees the sunlight was shining through gaps in the leaves and like normal. But the spots of light on the ground were all cresent shaped, like the Sun. So I expect that people underneath trees would notice that and look up and see that the Sun was crescent shaped and run around yelling that somethng was happening to the Sun. – M. A. Golding Mar 7 at 18:17
• @M.A.Golding Right, now I remember seeing that during the 2017 eclipse. Thanks. – Bob516 Mar 7 at 18:20

## 1 Answer

There are two ways to think about this change: firstly in terms of the amount of light energy. The rule for magnitudes is that "five magnitudes is a factor of 100" So to find the change in luminosity between two magnitudes you can calculate

$$\Delta L=100^{(M_1-M_2)/5}$$

With your values I get 0.47, or about half as bright at full eclipse.

The second way is to note that your eyes (and brain) actually do a good job of compensating for huge variation in brightness. Unless you are looking towards the sun (with your safety glasses of course) you probably won't notice the difference in illumination. In the 1999 eclipse, it was only at 90%+ coverage that there was any apparent change in illumination. The burghers of Vlissingen may have been blissfully unaware.

• I've been through two major partial eclipses and not only did it seem darker outside, the value of the highlights vs the shadows seemed very different. Then again I'm a photographer so maybe I notice that more than others. I also felt noticeably cooler during the eclipse. – Bob516 Mar 7 at 8:45
• The change in shadows was more noticiable for me than the change in illumination, but only (as I said) at much more than 50%. – James K Mar 7 at 8:58
• @Bob516 but you were looking for that. If you had no knowledge of the eclipse and you were going about your daily activities, you might not have noticed; your vision system, being relative and not an absolute instrument may not have flagged you that the amount of like was "wrong". – uhoh Mar 7 at 23:58
• @uhoh That is true, but this is all for a work of historical fiction I'm writing. I'm willing to stretch the truth about the level of the illumination. My protagonist will become aware there is an eclipse. A reader of my work might fact check me on the date and time of the eclipse. Seems unlikely they would check the percentage obscured of the eclipse, and if they did I would take out and wave around my artistic license. All to say, this completely answers my original question. – Bob516 Mar 8 at 1:13
• @Bob516 oh got it! Thanks for the background info, it makes the question much more interesting now. – uhoh Mar 8 at 1:23