Astronomical Twilight

Observations for Night Vision should be taken when the sun contributes no light to the SQM's sky glow reading.  You have to wait until after
astronomical twilight for total natural darkness. 

 

For a custom calendar with 
moon phases and twilight times, see
 
www.sunrisesunset.com/usa/Indiana.asp
 

(For the "Display Twilight Times" option,
select "Astronomical Twilight.")

 

Linked website by Steve Edwards


Do we really have to wait that late, especially in the summer?

Astronomer Ferdinando Patat of the European Southern Observatory writes about using SQMs for Night Vision:

"To be on the safe side, one should always wait for the end of astronomical twilight (sun elevation <=-18deg),
since in these conditions, at least at zenith, one is sure the sun is not contributing.
More practically, one can relax this condition to -15 degrees, as you can see from the plots in our recent
paper. On the other hand, the choice also depends on the light pollution at your sites. In heavily polluted sites,
the contribution by the sun might become negligible (with respect to the artificial contribution) much earlier.
... In principle, you could verify this with a simple test.  At least on one good night (no clouds), you should prepare your instrument 
at sunset and start measuring at zenith as soon as the instrumental dynamical range allows it, and record the brightness
values as the sun goes down, taking note of the time each measurement is done. Keep doing this
into the astronomical twilight. Then, you will be able to produce plots like ours and you will be in the
condition of judging when the sun contribution becomes negligible.
...In a pollution-free site, the difference between nautical (-12) and astronomical (-18) twilight in the V passband is more than
a magnitude, i.e. indeed significant.
... More details on night sky brightness and a general introduction to the field are at 
http://www.eso.org/~fpatat/science/skybright/paperII.pdf."

Thank you, Nando.

See also http://xxx.lanl.gov/PS_cache/astro-ph/pdf/0604/0604128.pdf 


Another Way of Looking at Astronomical Twilight

The table below from the U.S. Naval Observatory lists the non-dark hours, beginning in the morning at astronomical twilight and ending that night at astronomical twilight.  Between these hours, the sun contributes light to the sky.  Observations for the Night Vision program should not be taken between these hours.  The times below are from a 24-hour clock (the first two digits are hours, the second two digits are minutes).  During Daylight Saving Time (DST), you add one hour to the values given below.  For example, if you were to observe on May 23, with DST in effect you should take a reading of the sky glow either after 2306 (11:06 p.m.) or before 0418 (4:18 a.m.).
           o  ,    o  ,                                 SOUTH BEND, INDIANA                          Astronomical Applications Dept.
Location: W086 15, N41 40                           Astronomical Twilight for 2006                     U. S. Naval Observatory        
                                                                                                       Washington, DC  20392-5420     
                                                         Eastern Standard Time                                                        
                                                                                                                                      
                                                                                                                                      
       Jan.       Feb.       Mar.       Apr.       May        June       July       Aug.       Sept.      Oct.       Nov.       Dec.  
Day Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End
     h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m
01  0632 1905  0622 1936  0548 2008  0453 2046  0354 2131  0308 2219  0305 2233  0346 2156  0432 2056  0509 1959  0543 1914  0613 1854
02  0632 1906  0621 1937  0546 2009  0451 2048  0353 2133  0307 2220  0305 2232  0347 2154  0434 2054  0510 1957  0544 1912  0614 1854
03  0632 1907  0620 1938  0545 2010  0449 2049  0351 2134  0306 2221  0306 2232  0349 2152  0435 2052  0511 1956  0545 1911  0615 1854
04  0633 1908  0619 1939  0543 2011  0447 2050  0349 2136  0305 2223  0307 2231  0350 2151  0436 2050  0513 1954  0546 1910  0616 1854
05  0633 1909  0618 1940  0541 2012  0445 2052  0347 2138  0304 2224  0308 2230  0352 2149  0438 2048  0514 1952  0547 1909  0617 1854
06  0633 1909  0617 1942  0540 2014  0443 2053  0345 2139  0303 2225  0309 2230  0354 2147  0439 2047  0515 1951  0548 1908  0618 1854
07  0633 1910  0616 1943  0538 2015  0441 2054  0344 2141  0303 2226  0310 2229  0355 2145  0440 2045  0516 1949  0549 1907  0619 1854
08  0633 1911  0615 1944  0536 2016  0439 2056  0342 2143  0302 2227  0311 2228  0357 2143  0442 2043  0517 1947  0550 1907  0619 1854
09  0633 1912  0614 1945  0535 2017  0437 2057  0340 2144  0302 2228  0312 2227  0358 2141  0443 2041  0518 1946  0551 1906  0620 1854
10  0632 1913  0613 1946  0533 2018  0435 2059  0338 2146  0301 2228  0314 2226  0400 2140  0444 2039  0519 1944  0553 1905  0621 1855
11  0632 1914  0612 1947  0531 2020  0433 2100  0337 2147  0301 2229  0315 2225  0401 2138  0446 2037  0520 1942  0554 1904  0622 1855
12  0632 1915  0611 1948  0530 2021  0431 2102  0335 2149  0300 2230  0316 2224  0403 2136  0447 2035  0521 1941  0555 1903  0623 1855
13  0632 1916  0610 1949  0528 2022  0429 2103  0333 2151  0300 2231  0317 2223  0405 2134  0448 2033  0522 1939  0556 1902  0623 1855
14  0632 1917  0608 1951  0526 2023  0427 2105  0332 2152  0300 2231  0319 2222  0406 2132  0449 2031  0524 1938  0557 1902  0624 1855
15  0631 1918  0607 1952  0524 2024  0425 2106  0330 2154  0259 2232  0320 2221  0408 2130  0450 2029  0525 1936  0558 1901  0625 1856
16  0631 1919  0606 1953  0523 2026  0423 2107  0328 2156  0259 2232  0321 2220  0409 2128  0452 2027  0526 1935  0559 1900  0625 1856
17  0631 1920  0605 1954  0521 2027  0421 2109  0327 2157  0259 2233  0323 2218  0411 2126  0453 2025  0527 1933  0600 1900  0626 1856
18  0630 1921  0603 1955  0519 2028  0419 2110  0325 2159  0259 2233  0324 2217  0412 2124  0454 2023  0528 1932  0601 1859  0626 1857
19  0630 1922  0602 1956  0517 2029  0417 2112  0324 2200  0259 2234  0326 2216  0414 2122  0455 2021  0529 1930  0602 1859  0627 1857
20  0630 1923  0601 1957  0515 2031  0415 2114  0322 2202  0259 2234  0327 2214  0415 2120  0457 2019  0530 1929  0603 1858  0628 1858
21  0629 1924  0559 1958  0513 2032  0413 2115  0321 2203  0300 2234  0329 2213  0417 2118  0458 2017  0531 1927  0604 1858  0628 1858
22  0629 1925  0558 2000  0512 2033  0411 2117  0319 2205  0300 2234  0330 2212  0418 2116  0459 2015  0532 1926  0605 1857  0629 1859
23  0628 1926  0557 2001  0510 2034  0410 2118  0318 2206  0300 2234  0332 2210  0420 2114  0500 2014  0533 1925  0606 1857  0629 1859
24  0628 1927  0555 2002  0508 2036  0408 2120  0317 2208  0300 2234  0333 2209  0421 2112  0501 2012  0534 1923  0607 1856  0630 1900
25  0627 1928  0554 2003  0506 2037  0406 2121  0315 2209  0301 2234  0335 2207  0423 2110  0502 2010  0535 1922  0608 1856  0630 1900
26  0626 1929  0552 2004  0504 2038  0404 2123  0314 2211  0301 2234  0336 2206  0424 2108  0504 2008  0537 1921  0609 1856  0630 1901
27  0626 1930  0551 2005  0502 2040  0402 2125  0313 2212  0302 2234  0338 2204  0425 2106  0505 2006  0538 1919  0610 1855  0631 1902
28  0625 1932  0549 2007  0500 2041  0400 2126  0312 2214  0302 2234  0339 2202  0427 2104  0506 2004  0539 1918  0611 1855  0631 1902
29  0624 1933             0458 2042  0358 2128  0311 2215  0303 2233  0341 2201  0428 2102  0507 2003  0540 1917  0612 1855  0631 1903
30  0623 1934             0456 2043  0356 2129  0310 2216  0304 2233  0343 2159  0430 2100  0508 2001  0541 1916  0613 1855  0632 1904
31  0623 1935             0455 2045             0309 2218             0344 2157  0431 2058             0542 1915             0632 1904

                                             Add one hour for daylight time, if and when in use.

Twilight: Before sunrise and again after sunset there are intervals of time, twilight, during which there is natural light provided by the upper atmosphere, which does receive direct sunlight and reflects part of it toward the Earth's surface. Some outdoor activities may be conducted without artificial illumination during these intervals, and it is useful to have some means to set limits beyond which a certain activity should be assisted by artificial lighting. The major determinants of the amount of natural light during twilight are the state of the atmosphere generally and local weather conditions in particular. Atmospheric conditions are best determined at the actual time and place of events. Nevertheless, it is possible to establish useful, though necessarily approximate, limits applicable to large classes of activities by considering only the position of the Sun below the local horizon. Reasonable and convenient definitions have evolved.

Civil twilight
is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions in the absence of moonlight or other illumination. In the morning before the beginning of civil twilight and in the evening after the end of civil twilight, artificial illumination is normally required to carry on ordinary outdoor activities. Complete darkness, however, ends sometime prior to the beginning of morning civil twilight and begins sometime after the end of evening civil twilight.

Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon. At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible, and the horizon is indistinct.

Astronomical twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 18 degrees below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening the Sun does not contribute to sky illumination; for a considerable interval after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is practically imperceptible.

http://aa.usno.navy.mil/faq/docs/RST_defs.html 

 

www.nightwise.org

Copyright 2009 Chuck Bueter.  All rights reserved.