RELIVING GALILEO'S OBSERVATIONS

The purpose of this page is to give upcoming dates and times on which it is possible to view astronomical events as they were seen and recorded by Galileo Galilei in his earliest telescopic observations. All dates and times are given in what is called Universal Time (UT). Local clocks are normally offset from UT by a fixed number of hours (sometimes causing the date to differ as well). It is not possible to observe all the events listed from any given location. You will need to consult an ephemeris software or website to see if the Moon or Jupiter will be visible from your home at a particular universal date and time.

The Moon

As explained on our Moon Page, to the best of modern knowledge, Galileo did not record the dates or times of any of his lunar observations. Nonetheless, the eminent lunar scientist Ewen Whitaker has assigned what he believes are the correct dates and times (to the nearest half hour) to the four engravings appearing in Sidereus Nuncius, as well as to seven small wash drawings. The (unsigned/undated) drawings are bound with a manuscript copy of Sidereus Nuncius, and are thought (but with no great certainty) to have been made by Galileo during the same period of time. Whitaker assigned numbers and letters to each of these images. One drawing is thought to be a depiction of star emerging from behind the Moon, as it may have been seen on the morning of January 19, 1610. All others are thought, by Whitaker, to date from 1609. Whitaker's dates and times have been widely accepted by historians (see, for example, Gingerich and Van Helden (2003)), but they are not necessarily correct.

The pattern of light and shadow seen along the Moon's terminator (the line separating light from shadow) repeats with some regularity. Lunar scientists use a parameter called the solar colongitude to describe the position of the terminator. It is measured on a scale of 0-360 degrees, and when it has a particular value, the Moon's terminator is at a particular position relative to the craters and other surface markings. The lighting near the Moon's equator will be quite similar to what it is on any other day with the same colongitude. For the match to be exact all the way from the Moon's north to south pole, a second parameter, the latitude of the subsolar point on the Moon, also has to agree.

The following table lists the dates and times assigned by Whitaker to each of the eleven images, and gives their solar colongitude. If you look at the Moon on one of the dates and times listed, you will be able to see it with the terminator at the same colongitude as it had at the moment when Whitaker says Galileo's drawings were made. The appearance of the terminator should be quite similar. If the solar latitude is also the same as the original one, then the pattern of light and shadow all along the terminator should be exactly the same as what Galileo saw. If the sun's latitude is different, the terminator will be twisted slightly relative to the craters, causing some variation in the details of what is in light and shadow at positions far from the Moon's equator. For larger versions of the thumbnail images please see the list of original sources on our Moon Page.

Recent/Upcoming Dates Corresponding(?) to Galileo's Moon Images

					Modern
			Original per Whitaker		Month I		Month II		Month III
Drawing ID	Image	Colongitude	Date	Solar Latitude	Date	Solar Latitude	Date	Solar Latitude	Date	Solar Latitude
F1		321.2	30 Nov 1609 15:00 UT	+1.0	10 Apr 2008 01:26 UT	+1.2	09 May 2008 13:30 UT	+1.5	08 Jun 2008 00:50 UT	+1.4
F2		322.2	30 Nov 1609 17:00 UT	+1.0	10 Apr 2008 03:25 UT	+1.2	09 May 2008 15:29 UT	+1.5	08 Jun 2008 02:49 UT	+1.4
E1		323.2	30 Nov 1609 19:00 UT	+1.0	10 Apr 2008 05:25 UT	+1.2	09 May 2008 17:29 UT	+1.5	08 Jun 2008 04:49 UT	+1.4
F3		334.1	01 Dec 1609 16:30 UT	+1.0	11 Apr 2008 02:52 UT	+1.3	10 May 2008 14:54 UT	+1.5	09 Jun 2008 02:13 UT	+1.4
F4		346.0	02 Dec 1609 16:00 UT	+1.0	12 Apr 2008 02:18 UT	+1.3	11 May 2008 14:19 UT	+1.5	10 Jun 2008 01:37 UT	+1.4
E2		358.2	03 Dec 1609 16:00 UT	+1.0	13 Apr 2008 02:14 UT	+1.3	12 May 2008 14:13 UT	+1.5	11 Jun 2008 01:30 UT	+1.4
E3		162.1	17 Dec 1609 04:00 UT	+0.6	26 Apr 2008 13:06 UT	+1.5	26 May 2008 00:43 UT	+1.5	24 Jun 2008 11:47 UT	+1.1
F5		162.1	17 Dec 1609 04:00 UT	+0.6	26 Apr 2008 13:06 UT	+1.5	26 May 2008 00:43 UT	+1.5	24 Jun 2008 11:47 UT	+1.1
E4		174.2	18 Dec 1609 04:00 UT	+0.6	27 Apr 2008 13:01 UT	+1.5	27 May 2008 00:37 UT	+1.5	25 Jun 2008 11:40 UT	+1.1
F6		175.2	18 Dec 1609 06:00 UT	+0.5	27 Apr 2008 15:00 UT	+1.5	27 May 2008 02:36 UT	+1.5	25 Jun 2008 13:39 UT	+1.1
F7		204.2	19 Jan 1610 05:50 UT	-0.3	30 Apr 2008 00:01 UT	+1.5	29 May 2008 11:33 UT	+1.5	27 Jun 2008 22:34 UT	+1.1

Please note that these predictions refer only to the pattern of lighting and the position of the terminator relative to the surface features. Due to the Moon's rocking motions (called librations), the craters and other surface features may, on these modern dates, be displaced from the positions where Galileo would have been able to see them on the stated dates and times in 1609-1610. That is, the details of the pattern of light and shadow will be the same, but that pattern may be viewed from a somewhat different angle, giving the Moon a slightly different overall appearance. For example on 18 December 1609, the terminator was an almost perfectly straight line down the middle of the Moon. On the corresponding modern dates, the terminator will pass through the same craters (especially near the Moon's equator), but it may be displaced a little to the left or right.

We are personally unable to see any detailed resemblance between the expected appearance of the Moon on these dates, and the way it is depicted in Galileo's images, but this is based mostly on looking at photographs. We would be interested to know if any of you are able to see the resemblances imagined by Whitaker.

Please send your comments and observations to:

  jimmosher@yahoo.com

For those interested in the prediction software...

With the help and encouragement of Danish pathologist and Moon enthusiast Henrik Bondo, the program used for generating these predictions has been put into a form that should be fairly easy to use by any experienced lunar observer with a Windows PC. The program, which we call the "Lunar Terminator Visualization Tool" (LTVT) is freely downloadable from Henrik's "Lunar 100" website at:

  http://www.HenriksUCLA.dk

LTVT incorporates not only the program (used here) to predict modern dates and times when lighting conditions will duplicate those that could have been observed by Galileo; but also the program used on the Moon Page for graphically visualizing the expected terminator position by means of red and blue lines superimposed on a simulated image of the Moon. The graphic display includes a detailed interactive readout of the predicted altitude and azimuth of the Sun as seen from each visible lunar feature. It will also help you to learn the names of these features. Because windows controls have difficulty accepting dates before 1752, if you wish to simulate what Galileo would have seen on a particular date and time you will have to manually set the year to 1609 or 1610 using the Tools...SetYear... function. Also if you enter longitude=11.8715° (east) and latitude = 45.4003° (north) for the observer location, LTVT will tell you if the Moon and Sun would have been above the horizon as seen from Padua, Italy. The program used for producing the present predictions of modern times at which the lighting would be the same can be accessed through LTVT by clicking the Predict button in the lower right of the main screen. This will not only allow you to generate predictions covering a far greater range of time span than the ones listed here, but, if you enter your own location on the prediction page, LTVT will tell you whether the Moon and Sun will be visible from your location at those dates/times. LTVT also provides an easy means of searching through a user-supplied list of the dates and times of past photos/observations for ones with similar lighting.

The Janssen Region (?) -- Galileo's Triangular Peak

In Sidereus Nuncius, Galileo gives a vivid verbal description of sunrise over a triangular peak surrounded by three points of light and its gradual merging with the lighted side of the terminator. Galileo says he has depicted this event (which he saw towards the southern tip of the lunar terminator) in the engraving Whitaker calls E1. According to Whitaker, this sequence of events would have been seen by Galileo if he observed (from Padua, Italy) the central area of the crater Janssen on the evening of November 30, 1609, starting at sunset (15:30 UT) and ending at moonset (18:45 UT).

This image is a detail of engraving E1. For more information about Galileo's description and Ewen Whitaker's interpretation of it, please see our Moon Page. The triangular feature labeled "Wall" was, according to Whitaker, produced by light falling on the west wall of the crater Fabricius (which happens to lie near the center of the much larger crater Janssen). The items labeled 1, 2 and 3 are the three points of light mentioned by Galileo.

Note: due to the inevitable ambiguities of language, it is remotely possible that Galileo chose to depict the triangular peak after it had merged with the terminator, and that it is represented by the feature labeled "scarp". In this interpretation, "wall" would be one of the three points of light.

The lighting over a lunar feature can be described quite precisely by specifying the Sun's altitude above the horizontal (as seen from that feature) and its aziumth (measured clockwise from lunar north). Whenever those two parameters match, the same pattern of sunlit peaks and shaded valleys will be observed. Taking the lunar coordinates of the central region of Janssen as being 41° East/ 44° South, from 15:30 to 18:45 UT on November 30, 1609, the Sun's altitude over this point would have varied from +1.0° to +2.2° and its azimuth from 87.6° to 86.4°. the following table lists recent and upcoming dates and times at which the Sun's altitude over this point matches the values that would have been observed in 1609. The similarity in lighting is especially close if the azimuth matches as well. However the azimuth will not match precisely because the Sun's latitude north or south of the Moon's equator (and hence the exact colongitude required to produce a given solar altitude) varies. On November 30, 1609, the Sun's latitude on the Moon was +1.0°. Although variations of a few degrees in the Sun's azimuth rarely have a substantial effect on the appearance of lunar surface features, it will be seen from the azimuth data that the similarity to the 1609 values is closest (as it should be) on modern dates when the Sun's latitude is near +1.0°.

As noted in connection with the preceding predictions, although the table correctly predicts modern dates on which the lighting pattern over Janssen is expected to match that which Galileo would have seen if he was looking during the hours specified, the geometric appearance of this portion of the Moon may be somewhat different. This is because variations in the Moon's librations will cause Janssen to be seen closer or farther from the Moon's east limb, and also closer or farther from the Moon's southern limb. Because the Moon is spherical, this means the region may be seen with more or less foreshortening, and hence the same lighting pattern may be seen a bit more compressed or stretched out. The differences in foreshortening do not affect in any perceptible way the sequence of lighting events that would be observed.

These predictions are provided as a convenience for those wishing to observe the Moon under lighting conditions matching those specified by Whitaker. As explained on our Moon Page, we have good reason to suspect that this region of the Moon will not look like Galileo's engraving at these dates and times. We suspect you will find that the central region of Janssen will be fully illuminated, and that to actually see sunrise over the central "peak" you have to look several hours earlier. Even then, we do not believe it resembles the drawing. Moreover, and perhaps more importantly, the sequence of events that Galileo describes verbally in connection with the triangular peak seems impossibly long to have been observed in the three hours available to him from sunset to moonset on November 30, 1609.

We would be most interested in hearing from anyone with an alternative candidate for Galileo's triangular peak surrounded by three points of light. Again, please send your suggestions to:

  jimmosher@yahoo.com

Jupiter

The pattern of Jupiter's four Galilean satellites is constantly changing. Only rarely does a particular pattern recur, and even then not exactly. For example, even if the satellites are at the same positions from east to west, they may be at slightly different positions above and below the orbital plane. We would be interested in hearing from anyone who has the capability of predicting future dates on which the pattern of Jupiter's four Galilean satellites closely resembles that which would have been seen by Galileo when he first looked at them on the evening of January 7, 1610.

References :

1. O. Gingerich and A. Van Helden, "From Occhiale to Printed Page: The Making of Galileo's Sidereus Nuncius," Journal for the History of Astronomy 34, 251-267 (2003). (freely available from NASA's ADS)
2. E. Whitaker, "Galileo's Lunar Observations and the Dating of 'Sidereus Nuncius,'" Journal for the History of Astronomy 9, 155-169 (1978). (freely available from NASA's ADS)
3. E. Whitaker, Mapping And Naming The Moon: A History Of Lunar Cartography And Nomenclature (Cambridge University Press, 1999).    (Sample pages from this book may be viewed on Google Books.)

Images (unless otherwise credited) © Tom Pope and Jim Mosher
Last modified: May 9, 2008