Abstract

The sky brightness was measured during the partial phases and during totality of the 21 August 2017 total solar eclipse. A tracking CCD camera with color filters and a wide-angle lens allowed measurements across a wide field of view, recording images every 10 s. The partially and totally eclipsed Sun was kept behind an occulting disk attached to the camera, allowing direct brightness measurements from 1.5° to 38° from the Sun. During the partial phases, the sky brightness as a function of time closely followed the integrated intensity of the unobscured fraction of the solar disk. A redder sky was measured close to the Sun just before totality, caused by the redder color of the exposed solar limb. During totality, a bluer sky was measured, dimmer than the normal sky by a factor of 10,000. Suggestions for enhanced measurements at future eclipses are offered.

© 2018 Optical Society of America

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Summary of Sky Brightness Measurements During Eclipses of the Sun

W. E. Sharp, S. M. Silverman, and J. W. F. Lloyd
Appl. Opt. 10(6) 1207-1210 (1971)

References

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  1. W. E. Sharp, J. W. F. Lloyd, and S. M. Silverman, “Zenith skylight intensity and color during the total solar eclipse of 20 July 1963,” Appl. Opt. 5, 787–792 (1966).
    [Crossref]
  2. B. S. Dandekar, “Measurements of the zenith sky brightness and color during the total solar eclipse of 12 November 1966 at Quehua, Bolivia,” Appl. Opt. 7, 705–710 (1968).
    [Crossref]
  3. W. E. Sharp, S. M. Silverman, and J. W. F. Lloyd, “Summary of sky brightness measurements during eclipses of the Sun,” Appl. Opt. 10, 1207–1210 (1971).
    [Crossref]
  4. D. A. Velasquez, “Zenith sky brightness and color change during the total solar eclipse of 12 November 1966 at Santa Ines, Peru,” Appl. Opt. 10, 1211–1214 (1971).
    [Crossref]
  5. G. P. Können and C. Hinz, “Visibility of stars, halos, and rainbows during solar eclipses,” Appl. Opt. 47, H14–H24 (2008).
    [Crossref]
  6. J. W. F. Lloyd and S. M. Silverman, “Measurements of the zenith sky intensity and spectral distribution during the solar eclipse of 12 November 1966 at Bage, Brazil, and on an aircraft,” Appl. Opt. 10, 1215–1219 (1971).
    [Crossref]
  7. B. S. Dandekar and J. P. Turtle, “Day sky brightness and polarization during the total solar eclipse of 7 March 1970,” Appl. Opt. 10, 1220–1224 (1971).
    [Crossref]
  8. G. E. Shaw, “Sky radiance during a total eclipse: a theoretical model,” Appl. Opt. 17, 272–276 (1978).
    [Crossref]
  9. S. Nawar, A. B. Morcos, and J. S. Mikhail, “Photoelectric study of the sky brightness along Sun’s meridian during the March 29, 2006 solar eclipse,” New Astron. 12, 562–568 (2007).
    [Crossref]
  10. W. F. H. Moll, H. C. Burger, and F. van der Bilt, “The distribution of energy over the Sun’s disc,” Bull. Astron. Inst. Neth. 3, 83–89 (1925).
  11. P. Koepke, J. Reuder, and J. Schween, “Spectral variation of the solar radiation during an eclipse,” Meteorol. Z. 10, 179–186 (2001).
    [Crossref]
  12. C. Emde and B. Mayer, “Simulation of solar radiation during a total eclipse: a challenge for radiative transfer,” Atmos. Chem. Phys. 7, 2259–2270 (2007).
    [Crossref]
  13. S. D. Gedzelman, “Sky color near the horizon during a total solar eclipse,” Appl. Opt. 14, 2831–2837 (1975).
    [Crossref]
  14. K.-P. Möllmann and M. Vollmer, “Measurements and predictions of the illuminance during a solar eclipse,” Eur. J. Phys. 27, 1299–1314 (2006).
    [Crossref]
  15. D. G. Bruns, “Gravitational starlight deflection measurements during the 21 August 2017 total solar eclipse,” Class. Quantum Grav. 35, 075009 (2018).
    [Crossref]

2018 (1)

D. G. Bruns, “Gravitational starlight deflection measurements during the 21 August 2017 total solar eclipse,” Class. Quantum Grav. 35, 075009 (2018).
[Crossref]

2008 (1)

2007 (2)

S. Nawar, A. B. Morcos, and J. S. Mikhail, “Photoelectric study of the sky brightness along Sun’s meridian during the March 29, 2006 solar eclipse,” New Astron. 12, 562–568 (2007).
[Crossref]

C. Emde and B. Mayer, “Simulation of solar radiation during a total eclipse: a challenge for radiative transfer,” Atmos. Chem. Phys. 7, 2259–2270 (2007).
[Crossref]

2006 (1)

K.-P. Möllmann and M. Vollmer, “Measurements and predictions of the illuminance during a solar eclipse,” Eur. J. Phys. 27, 1299–1314 (2006).
[Crossref]

2001 (1)

P. Koepke, J. Reuder, and J. Schween, “Spectral variation of the solar radiation during an eclipse,” Meteorol. Z. 10, 179–186 (2001).
[Crossref]

1978 (1)

1975 (1)

1971 (4)

1968 (1)

1966 (1)

1925 (1)

W. F. H. Moll, H. C. Burger, and F. van der Bilt, “The distribution of energy over the Sun’s disc,” Bull. Astron. Inst. Neth. 3, 83–89 (1925).

Bruns, D. G.

D. G. Bruns, “Gravitational starlight deflection measurements during the 21 August 2017 total solar eclipse,” Class. Quantum Grav. 35, 075009 (2018).
[Crossref]

Burger, H. C.

W. F. H. Moll, H. C. Burger, and F. van der Bilt, “The distribution of energy over the Sun’s disc,” Bull. Astron. Inst. Neth. 3, 83–89 (1925).

Dandekar, B. S.

Emde, C.

C. Emde and B. Mayer, “Simulation of solar radiation during a total eclipse: a challenge for radiative transfer,” Atmos. Chem. Phys. 7, 2259–2270 (2007).
[Crossref]

Gedzelman, S. D.

Hinz, C.

Koepke, P.

P. Koepke, J. Reuder, and J. Schween, “Spectral variation of the solar radiation during an eclipse,” Meteorol. Z. 10, 179–186 (2001).
[Crossref]

Können, G. P.

Lloyd, J. W. F.

Mayer, B.

C. Emde and B. Mayer, “Simulation of solar radiation during a total eclipse: a challenge for radiative transfer,” Atmos. Chem. Phys. 7, 2259–2270 (2007).
[Crossref]

Mikhail, J. S.

S. Nawar, A. B. Morcos, and J. S. Mikhail, “Photoelectric study of the sky brightness along Sun’s meridian during the March 29, 2006 solar eclipse,” New Astron. 12, 562–568 (2007).
[Crossref]

Moll, W. F. H.

W. F. H. Moll, H. C. Burger, and F. van der Bilt, “The distribution of energy over the Sun’s disc,” Bull. Astron. Inst. Neth. 3, 83–89 (1925).

Möllmann, K.-P.

K.-P. Möllmann and M. Vollmer, “Measurements and predictions of the illuminance during a solar eclipse,” Eur. J. Phys. 27, 1299–1314 (2006).
[Crossref]

Morcos, A. B.

S. Nawar, A. B. Morcos, and J. S. Mikhail, “Photoelectric study of the sky brightness along Sun’s meridian during the March 29, 2006 solar eclipse,” New Astron. 12, 562–568 (2007).
[Crossref]

Nawar, S.

S. Nawar, A. B. Morcos, and J. S. Mikhail, “Photoelectric study of the sky brightness along Sun’s meridian during the March 29, 2006 solar eclipse,” New Astron. 12, 562–568 (2007).
[Crossref]

Reuder, J.

P. Koepke, J. Reuder, and J. Schween, “Spectral variation of the solar radiation during an eclipse,” Meteorol. Z. 10, 179–186 (2001).
[Crossref]

Schween, J.

P. Koepke, J. Reuder, and J. Schween, “Spectral variation of the solar radiation during an eclipse,” Meteorol. Z. 10, 179–186 (2001).
[Crossref]

Sharp, W. E.

Shaw, G. E.

Silverman, S. M.

Turtle, J. P.

van der Bilt, F.

W. F. H. Moll, H. C. Burger, and F. van der Bilt, “The distribution of energy over the Sun’s disc,” Bull. Astron. Inst. Neth. 3, 83–89 (1925).

Velasquez, D. A.

Vollmer, M.

K.-P. Möllmann and M. Vollmer, “Measurements and predictions of the illuminance during a solar eclipse,” Eur. J. Phys. 27, 1299–1314 (2006).
[Crossref]

Appl. Opt. (9)

W. E. Sharp, J. W. F. Lloyd, and S. M. Silverman, “Zenith skylight intensity and color during the total solar eclipse of 20 July 1963,” Appl. Opt. 5, 787–792 (1966).
[Crossref]

B. S. Dandekar, “Measurements of the zenith sky brightness and color during the total solar eclipse of 12 November 1966 at Quehua, Bolivia,” Appl. Opt. 7, 705–710 (1968).
[Crossref]

S. D. Gedzelman, “Sky color near the horizon during a total solar eclipse,” Appl. Opt. 14, 2831–2837 (1975).
[Crossref]

G. E. Shaw, “Sky radiance during a total eclipse: a theoretical model,” Appl. Opt. 17, 272–276 (1978).
[Crossref]

W. E. Sharp, S. M. Silverman, and J. W. F. Lloyd, “Summary of sky brightness measurements during eclipses of the Sun,” Appl. Opt. 10, 1207–1210 (1971).
[Crossref]

D. A. Velasquez, “Zenith sky brightness and color change during the total solar eclipse of 12 November 1966 at Santa Ines, Peru,” Appl. Opt. 10, 1211–1214 (1971).
[Crossref]

J. W. F. Lloyd and S. M. Silverman, “Measurements of the zenith sky intensity and spectral distribution during the solar eclipse of 12 November 1966 at Bage, Brazil, and on an aircraft,” Appl. Opt. 10, 1215–1219 (1971).
[Crossref]

B. S. Dandekar and J. P. Turtle, “Day sky brightness and polarization during the total solar eclipse of 7 March 1970,” Appl. Opt. 10, 1220–1224 (1971).
[Crossref]

G. P. Können and C. Hinz, “Visibility of stars, halos, and rainbows during solar eclipses,” Appl. Opt. 47, H14–H24 (2008).
[Crossref]

Atmos. Chem. Phys. (1)

C. Emde and B. Mayer, “Simulation of solar radiation during a total eclipse: a challenge for radiative transfer,” Atmos. Chem. Phys. 7, 2259–2270 (2007).
[Crossref]

Bull. Astron. Inst. Neth. (1)

W. F. H. Moll, H. C. Burger, and F. van der Bilt, “The distribution of energy over the Sun’s disc,” Bull. Astron. Inst. Neth. 3, 83–89 (1925).

Class. Quantum Grav. (1)

D. G. Bruns, “Gravitational starlight deflection measurements during the 21 August 2017 total solar eclipse,” Class. Quantum Grav. 35, 075009 (2018).
[Crossref]

Eur. J. Phys. (1)

K.-P. Möllmann and M. Vollmer, “Measurements and predictions of the illuminance during a solar eclipse,” Eur. J. Phys. 27, 1299–1314 (2006).
[Crossref]

Meteorol. Z. (1)

P. Koepke, J. Reuder, and J. Schween, “Spectral variation of the solar radiation during an eclipse,” Meteorol. Z. 10, 179–186 (2001).
[Crossref]

New Astron. (1)

S. Nawar, A. B. Morcos, and J. S. Mikhail, “Photoelectric study of the sky brightness along Sun’s meridian during the March 29, 2006 solar eclipse,” New Astron. 12, 562–568 (2007).
[Crossref]

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Figures (8)

Fig. 1.
Fig. 1. Commercial Meade equatorial tracking mount supported a Diffraction Limited/SBIG ST-I-C CCD camera and a small obscuring disk. The mount was set up in equatorial mode during the eclipse to accurately track the Sun.
Fig. 2.
Fig. 2. Dashed blue curve indicates the quantum efficiency of the blue-filtered pixels, multiplied by the transmission of the external BG-38 filter. The solid red curve indicates the efficiency of the red-filtered pixels multiplied by the BG-38 transmission. The red efficiency was significantly attenuated at the long wavelengths by the BG-38 filter. The green-filtered pixels were not analyzed.
Fig. 3.
Fig. 3. Three-second image taken during totality shows the obscuring disk and its support bar reaching down from the top center. The color has been corrected according to the curves in Fig. 2 to indicate the true color measured by the sensor. Detailed analyses of pixel intensity were performed along the white rectangle, averaging each column of 20 pixels at all distances from the Sun. Vertical bars are placed at 5° and 25° from the Sun to indicate the angular scale.
Fig. 4.
Fig. 4. Simulated eclipse images were prepared using a black mask to accurately include monochromatic limb darkening. The top image is at 21 s before C2, the middle at 70 s before C2, and the lower image at 225 s before C2. The corona was not included because it was much dimmer than the exposed solar limb.
Fig. 5.
Fig. 5. Normalized red and blue image intensities and the visible Sun as a function of time. Data measured at 25° from the Sun (not shown here) appear identical on this scale.
Fig. 6.
Fig. 6. Ratio of the calibrated intensity in the red-filtered pixels to the calibrated intensity in the blue-filtered pixels is plotted as a function of angular distance from the Sun for the three times shown in Fig. 4. The curves for C2 (70 s) and C2 (21 s) are essentially identical. The curve for the C2 (225 s) is consistently slightly lower. These data are replotted in Fig. 7 using a different horizontal axis.
Fig. 7.
Fig. 7. Ratio of the calibrated intensity in the red-filtered pixels to the calibrated intensity in the blue-filtered pixels is plotted as a function of time before C2 for four angular distances from the Sun. The color tends toward red at all angles, with a larger trend for angles closer to the Sun. The top curve is for the ratio at 1.5°, and the bottom curve is for the ratio at 12°.
Fig. 8.
Fig. 8. Normalized red and blue intensities during totality. These data are the same as for Fig. 5, except that the scales are expanded and details for measurements farther from the Sun are included.