Why pictures of the pale blue dot ana lemma look like movement of measured measure meant to mean meaning
An analemma is that figure-8 curve you get when you mark the position of the Sun at the same time each day for one year. But the trick to imaging an analemma of the Moon is to wait bit longer. On average the Moon returns to the same position in the sky about 50 minutes and 29 seconds later each day. So photograph the Moon 50 minutes 29 seconds later on successive days. Over one lunation or lunar month it will trace out an analemma-like curve as the Moon's actual position wanders due to its tilted and elliptical orbit. To create this composite image of a lunar analemma, astronomer Gyorgy Soponyai chose a lunar month from March 26 to April 18 with a good stretch of weather and a site close to home near Mogyorod, Hungary. Crescent lunar phases too thin and faint to capture around the New Moon are missing though. Facing southwest, the lights of Budapest are in the distance of the base image taken on March 27.
Image Credit & Copyright: Gyorgy Soponyai
Specific copyrights apply.
Analemma
In astronomy, an analemma (/ˌænəˈlɛmə/; from Ancient Greek ἀνάλημμα (analēmma) 'support')[a] is a diagram showing the position of the Sun in the sky as seen from a fixed location on Earth at the same mean solar time, as that position varies over the course of a year. The diagram will resemble a figure eight. Globes of Earth often display an analemma as a two-dimensional figure of equation of time vs. declination of the Sun.
The north–south component of the analemma results from the change in the Sun's declination due to the tilt of Earth's axis of rotation. The east–west component results from the nonuniform rate of change of the Sun's right ascension, governed by the combined effects of Earth's axial tilt and its orbital eccentricity.[1]
One can photograph an analemma by keeping a camera at a fixed location and orientation and taking multiple exposures throughout the year, always at the same time of day (disregarding daylight saving time).
Analemmas (in the modern sense of the term) have been used in conjunction with sundials since the 18th century to convert between apparent and mean solar time. Before this, the term had a more generic meaning that referred to a graphical procedure of representing three-dimensional objects in two dimensions, now known as orthographic projection.[2][3]
Although the term analemma usually refers to Earth's solar analemma, it can be applied to other celestial bodies as well.
Description[edit source]
An analemma can be traced by plotting the position of the Sun as viewed from a fixed position on Earth at the same clock time every day for an entire year, or by plotting a graph of the Sun's declination against the equation of time. The resulting curve resembles a long, slender figure-eight with one lobe much larger than the other. This curve is commonly printed on terrestrial globes, usually in the eastern Pacific Ocean, the only large tropical region with very little land. It is possible, though challenging, to photograph the analemma, by leaving the camera in a fixed position for an entire year and snapping images on 24-hour intervals (or some multiple thereof); see section below.
The long axis of the figure—the line segment joining the northernmost point on the analemma to the southernmost—is bisected by the celestial equator, to which it is approximately perpendicular, and has a "length" of twice the obliquity of the ecliptic, i.e., about 47°. The component along this axis of the Sun's apparent motion is a result of the familiar seasonal variation of the declination of the Sun through the year. The "width" of the figure is due to the equation of time, and its angular extent is the difference between the greatest positive and negative deviations of local solar time from local mean time when this time-difference is related to angle at the rate of 15° per hour, i.e., 360° in 24 h. This width of the analemma is approximately 7.7°, so the length of the figure is more than six times its width. The difference in size of the lobes of the figure-eight form arises mainly from the fact that the perihelion and aphelion occur far from the equinoxes. They also occur a mere couple of weeks after solstices, which in turn causes slight tilt of the figure eight and its minor lateral asymmetry.
There are three parameters that affect the size and shape of the analemma—obliquity, eccentricity, and the angle between the northward equinox and the periapsis. Viewed from an object with a perfectly circular orbit and no axial tilt, the Sun would always appear at the same point in the sky at the same time of day throughout the year and the analemma would be a dot. For an object with a circular orbit but significant axial tilt, the analemma would be a figure of eight with northern and southern lobes equal in size. For an object with an eccentric orbit but no axial tilt, the analemma would be a straight east–west line along the celestial equator.[1]
The north–south component of the analemma shows the Sun's declination, its latitude on the celestial sphere, or the latitude on the Earth at which the Sun is directly overhead. The east–west component shows the equation of time, or the difference between solar time and local mean time. This can be interpreted as how "fast" or "slow" the Sun (or an analemmatic sundial) is compared to clock time. It also shows how far west or east the Sun is, compared with its mean position. The analemma can be considered as a graph in which the Sun's declination and the equation of time are plotted against each other. In many diagrams of the analemma, a third dimension, that of time, is also included, shown by marks that represent the position of the Sun at various, fairly closely spaced, dates throughout the year.
In diagrams, the analemma is drawn as it would be seen in the sky by an observer looking upward. If north is at the top, west is to the right. This corresponds with the sign of the equation of time, which is positive in the westward direction. The further west the Sun is, compared with its mean position, the more "fast" a sundial is, compared with a clock. (See Equation of time#Sign of the equation of time.) If the analemma is a graph with positive declination (north) plotted upward, positive equation of time (west) is plotted to the right. This is the conventional orientation for graphs. When the analemma is marked on a geographical globe, west in the analemma is to the right, while the geographical features on the globe are shown with west to the left. To avoid this confusion, it has been suggested that analemmas on globes should be printed with west to the left, but this is not done, at least, not frequently. In practice, the analemma is so nearly symmetrical that the shapes of the mirror images are not easily distinguished, but if date markings are present, they go in opposite directions. The Sun moves eastward on the analemma near the solstices. This can be used to tell which way the analemma is printed. See the image above, at high magnification.
An analemma that includes an image of a solar eclipse has been called a tutulemma, a term coined by photographers Cenk E. Tezel and Tunç Tezel based on the Turkish word for eclipse.[4
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