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I am looking for a way find the time when:

  1. Pleiades will be at the Zenith today (11th November 2011)
  2. Pleiades will conjunct with the Moon
  3. The Moon will be at the Zenith today

How do I find those out?

Ingress and Egress would be helpful too. I live in Asia/Rangoon.

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2 Answers 2

up vote 3 down vote accepted

An object will only cross the zenith if you happen to live at a latitude equal to its declination (for the Pleiades, that is 27 degrees, 7 arcminutes N, from Wikipedia). Otherwise, it will miss the zenith by the same angle as the difference between your location and its declination.

It will be at the highest it happens to reach when the local sidereal time is equal to its right ascension, in an event called transitting (for the Pleiades, at 03:47 local sidereal time). Ordinary wall clock/wristwatch time coincides with local sidereal time on the Vernal Equinox, and then the sidereal time get ahead of ordinary time by 3 minutes and 56 seconds every subsequent day, or roughly two hours per month. That means for November, that sidereal time is about 16 hours ahead, so the Pleiades will transit something like 03:47 minus 16 hours = about noon, local time. You can calculate your exact local sidereal time with any number of websites if you know your exact longitude. There was an Android app that would use your GPS to determine it automatically, but the latest version is broken and does not work at all, on my phone at least.

You can do a similar calculation for the Moon using its phase, but the easiest thing to do is just play with some planetarium software until you see the event yourself.

Hang on, is this for a Muslim religious observation? This has been discussed elsewhere on this site before, so you might look for it. Officially, I believe the proper time of observation is strictly experimentally determined, that is, when the Moon is actually spotted by some properly qualified religious figure, probably observing from Mecca.

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Thanks. It is for a Jyotish purpose. This particular time is considered most potent. The Krritika Zenith. As the Vedic magazine has stopped publishing these timings, I was asked to use the "Internet" to find it out. I tried with Google Sky on Android as well, but could not get it. –  Ravi Chhabra Nov 10 '11 at 15:00
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If you know the day, approximately, then install Stellarium on your computer.

It needs to be calibrated after installation, but that's something you do only once and then you can forget about it: hover the mouse on the bottom-left side, click the Location Window button (or hit F6), then enter your present location or a big city nearby. That's it, now it's calibrated.

Then click the Date/Time Window button, enter the known date, and tweak the time dial until the Moon is right above the S mark (the south direction). You can drag the map with the mouse, zoom in and out, etc.

Play with the other settings, disable the Atmosphere if you want to see the stars during daylight time, etc.

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