A lamp to tell you where you are

Evening sky scene 2016 July 9 from longitude 90 west

The Moon – like me and like the United States – celebrated its birthday on July 4 – that is, was New.

So now this Moon, learning to walk and talk three hundred times more rapidly than any human baby, emerges into the evening sky, where the background awaiting it at this time of year is the starry region of Leo.  It passes about three finger-thicknesses (one and a half degrees) south of Regulus on July 7 (at 23 Universal Time), then ascends through the ecliptic plane (July 9, about 2 UT), then much closer to Jupiter, in fact in front of it as seen from our far southern hemisphere.  The time of this conjunction is July 9 at 10 UT, which is daytime for America and Europe, so the occultation is observable only from the ocean south of Australia.

Occultation of Jupiter 2016 July 9

Detail from the Occultations section in Astronomical Calendar 2016.

We on the other side of the Earth see the Moon to the west of the planet in the evening sky of July 8, then to the east of it in the evening of July 9.

I’m going to show this scene with the Moon at true size instead of exaggerated as usual, and from two localities: on the longitude 0 line (the prime meridian that runs through Greenwich) –

Evening sky scene 2016 Kuly 8 from longitude zero

– and at longitude 90 degrees west, in the middle of the U.S.

Evening sky scene 2016 July 9 from longitude 90 west

But both at latitude 40 north for simplicity.

(Don’t forget to right-click so as to see the images larger.)

The Moon has to appear small in the pictures – because it is at true scale, and because it is Young and therefore slender at this time, and because the picture has to be large enough to span the other relevant parts of the scene – but I hope you can see it.  Its “age” from the New Moon moment is 81 hours in the first picture and 87 in the second.

You should be able to see the change in its position, in the 6 hours from the longitude-zero scene to the longitude-90 scene.  It has moved forward about 3.3 degrees relative to Regulus and Jupiter.

We can turn this around.  If you don’t know where the heck you are, but you see the Moon at this position at this time, you should be able to work out that you are 90 degrees west of the Greenwich meridian.  You would work it out by knowing where the Moon should be as seen at this time from the Greenwich meridian.

That was just what the navigators of the seventeenth century needed to know in order to determine their longitudes at sea, and it was why the astronomers set up the Greenwich Observatory, hoping to use the Moon in just this way to provide them with the means of knowing.  The means were elaborate, and have since then become the body of observations and mathematics behind our computer programming that gives the answer in an eyeblink.  If a ship’s captain of the time had had all that, he would have been able to know instantly that he had sailed to Springfield, Illinois – quite a feat.


4 thoughts on “A lamp to tell you where you are”

  1. I’ve always been a fan of new/old moon spotting; even got my younger brother into looking for ’em too. I’ve seen crescents with tips broken into dots, somewhat reminiscent of bailey’s beads during total solar eclipses. Too bad weather’s been uncooperative here in NY lateley for this latest opportunity, and once again I’d like to add this link for all you other moon watchers out there……


    Have fun, keep cool, and safe.

  2. I’ve been studying celestial navigation lately. It’s fascinating.

    Tuesday evening was too cloudy to try to find the very young crescent Moon after sunset. Yesterday I went outside at sunset and saw her above the roof of the church across the street, farther north than I expected.

    Because the waxing crescent Moon was visible, Islamic traditionalists celebrated Eid yesterday evening. I think most Muslims celebrated Eid the night before, based on a prediction of when the waxing crescent Moon would (or should) be visible in Saudi Arabia.

    1. Or it is based on an announcement coming from the Grand Mufti of Egypt. But various Muslim countries use different ways of defining the beginning of the month.

      1. Thanks. My Muslim coworker is back at work today, so Eid must have been yesterday. That’s how I can tell.

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