NavList:

For the next couple of weeks, look for Venus in daylight at about 9:08 Local Mean Time. You'll find Venus very near the meridian. Its altitude will be almost the same as Sirius at meridian passage, but Venus will be climbing gradually as the month continues. You can use Sirius to select a good observing spot. Find out when Sirius is right on the meridian for your location. Then find a spot where you can stand with Sirius nicely aligned with some foreground feature, like the top of a tree or a roofline. Mark that place on the ground with a couple of rocks, so that you can stand in exactly the same location in the next ten days or so in daylight. I just did this a few minutes ago. If you have neighbors or family who like to tidy up, ask them not to move your markers!

So when is 9:08 Local Mean Time for your location?? Here in most of the US and Canada, we begin DST, "springing ahead" by an hour, just about 30 hours from now as I type this. That complicates matters. The easy way around this is to use GMT (UT). At Greenwich, on the prime meridian, Venus will be above that meridian at 9:08 GMT. Because GMT is LMT on the prime meridian. How long does Venus take to get your meridian? It's just the rotation of the Earth. Convert your longitude to hours and minutes --every 15° of longitude is one hour and every extra degree is four minutes-- and add that on to 9:08 GMT. That's when you go out to look for Venus on your own meridian (conveniently bypassing zone time, zone time offsets, and daylight saving/summer time issues entirely). For example, suppose your longitude is 70°W. The multiple of 15 that's less than 70 is 60 so we get four hours for those 4·15 degrees. The ten degrees remaining above 60 each give four minutes, so that's 40 minutes. The grand total is 4h40m which we add on to 9:08 GMT. Thus at longitude 70°W, Venus would transit the meridian at 13:48 GMT. Another example: suppose your longitude is 92.5°W. The nearest multiple of 15 below this is 90. We get six hours from that. The remainder of 2.5° multiplied by four is 10 minutes. So the total is 6h10m and therefore at longitude 92.5°W, Venus will be on the meridian at 15:18 GMT. For a final example, let's go east. Suppose your longitude is 37.5°E. That's equivalent to 2h30m, but if we're east of Greenwich, meridian passage occurs earlier (when counted by GMT). So at longitude 37.5°E, Venus will be on the meridian at 06:38 GMT. Don't get carried away with the calculating. This is just an approximate time anway, good to the nearest few minutes.

It's not actually necessary to do this at meridian passage, but there are advantages. Being aligned North/South means there's a good chance you'll have an available building or road or similar aligned to the compass points so that you can identify the meridian direction easily. And of course Venus will be at its highest altitude at that time, which implies somewhat darker background blue sky and less absorption of the light of Venus itself by the atmosphere. I recommend looking for Venus with binoculars on the first try. You can see it with your own two eyes, "naked eye", after you've found it with better optics. Once you spot it naked eye, you'll be amazed how bright it is and surprised that you didn't see it right away.

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA