Solar zenith angles for biological research

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Description

This function calculates the solar zenith, azimuth and declination angles, time at sunrise, local noon and sunset, day length, and PAR (photosynthetically available radiation, 400-700 nm) under clear skies and average atmospheric conditions (marine or continental) anywhere on the surface of the earth based on date, time, and location.

Usage

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astrocalc4r(day, month, year, hour, timezone, lat, lon, withinput = FALSE, 
seaorland = "maritime", acknowledgment = FALSE)

Arguments

day

day of month in the local time zone (integers). Value is required. Multiple observations should be enclosed with the c() function.

month

month of year in the local time zone (integers). Value is required. Multiple observations should be enclosed with the c() function.

year

year in the local time zone (integers).Value is required. Multiple observations should be enclosed with the c() function.

hour

local time for each observation (decimal hours, e.g. 11:30 PM is 23.5, real numbers). Value is required. Multiple observations should be enclosed with the c() function.

timezone

local time zone in +/- hours relative to GMT to link local time and GMT. For example, the difference between Eastern Standard Time and GMT is -5 hours. Value is required. Multiple observations should be enclosed with the c() function. timezone should include any necessary adjustments for daylight savings time.

lat

Latitude in decimal degrees (0o to 90 o in the northern hemisphere and -90 o to 0 o degrees in the southern hemisphere, real numbers). For example, 42o 30' N is 42.5 o and 42o 30' S is -42.5o. Value is required. Multiple observations should be enclosed with the c() function.

lon

Longitude in decimal degrees (-0 o to 180 o in the western hemisphere and 0o to 180 o in the eastern hemisphere, real numbers). For example, 110o 15' W is -110.25 o and 110o 15' E is 110.25o. Value is required. Multiple observations should be enclosed with the c() function.

withinput

logical:TRUE to return results in a dataframe with the input data; otherwise FALSE returns a dataframe with just results. Default is FALSE.

seaorland

text: "maritime" for typical maritime conditions or "continental" for typical continental conditions. Users must select one option or the other based on proximity to the ocean or other factors.

acknowledgment

logical: use TRUE to output acknowledgement. Default is FALSE.

Details

Astronomical definitions are based on definitions in Meeus (2009) and Seidelmann (2006). The solar zenith angle is measured between a line drawn "straight up" from the center of the earth through the observer and a line drawn from the observer to the center of the solar disk. The zenith angle reaches its lowest daily value at local noon when the sun is highest. It reaches its maximum value at night after the sun drops below the horizon. The zenith angle and all of the solar variables calculated by astrocalc4r depend on latitude, longitude, date and time of day. For example, solar zenith angles measured at the same time of day and two different locations would differ due to differences in location. Zenith angles at the same location and two different dates or times of day also differ.

Local noon is the time of day when the sun reaches its maximum elevation and minimum solar zenith angle at the observers location. This angle occurs when the leading edge of the sun first appears above, or the trailing edge disappears below the horizon (0.83o accounts for the radius of the sun when seen from the earth and for refraction by the atmosphere). Day length is the time in hours between sunrise and sunset. Solar declination and azimuth angles describe the exact position of the sun in the sky relative to an observer based on an equatorial coordinate system (Meeus 2009). Solar declination is the angular displacement of the sun above the equatorial plane. The equation of time accounts for the relative position of the observer within the time zone and is provided because it is complicated to calculate. PAR isirradiance in lux (lx, approximately W m-2) at the surface of the earth under clear skies calculated based on the solar zenith angle and assumptions about marine or terrestrial atmospheric properties. astrocalc4r calculates PAR for wavelengths between 400-700 nm. Calculations for other wavelengths can be carried out by modifying the code to use parameters from Frouin et al. (1989). Following Frouin et al. (1989), PAR is assumed to be zero at solar zenith angles >= 90o although some sunlight may be visible in the sky when the solar zenith angle is < 108o. Angles in astrocalc4r output are in degrees although radians are used internally for calculations. Time data and results are in decimal hours (e.g. 11:30 pm = 23.5 h) local time but internal calculations are in Greenwich Mean Time (GMT). The user must specify the local time zone in terms of +/- hours relative to GMT to link local time and GMT. For example, the difference between Eastern Standard Time and GMT is -5 hours. The user must ensure that any adjustments for daylight savings time are included in the timezone value. For example, timezone=-6 for Eastern daylight time.

Value

Time of solar noon, sunrise and sunset, angles of azimuth and zenith, eqtime, declination of sun, daylight length (hours) and PAR.

Author(s)

Larry Jacobson, Alan Seaver, and Jiashen Tang NOAA National Marine Fisheries Service Northeast Fisheries Science Center, 166 Water St., Woods Hole, MA 02543

Larry.Jacobson@noaa.gov

References

Frouin, R., Lingner, D., Gautier, C., Baker, K. and Smith, R. 1989. A simple analytical formula to compute total and photosynthetically available solar irradiance at the ocean surface under clear skies. J. Geophys. Res. 94: 9731-9742.

L. D. Jacobson, L. C. Hendrickson, and J. Tang. 2015. Solar zenith angles for biological research and an expected catch model for diel vertical migration patterns that affect stock size estimates for longfin inshore squid (Doryteuthis pealeii). Canadian Journal of Fisheries and Aquatic Sciences 72: 1329-1338.

Meeus, J. 2009. Astronomical Algorithms, 2nd Edition. Willmann-Bell, Inc., Richmond, VA. Seidelmann, P.K. 2006. Explanatory Supplement to the Astronomical Almanac. University Science Books, Sausalito, CA.

Seidelmann, P.K. 2006. Explanatory Supplement to the Astronomical Almanac. University Science Books, Sausalito, CA. This function is an R implementation of:

Jacobson L, Seaver A, Tang J. 2011. AstroCalc4R: software to calculate solar zenith angle; time at sunrise, local noon and sunset; and photosynthetically available radiation based on date, time and location. US Dept Commer, Northeast Fish Sci Cent Ref Doc. 11-14; 10 p. Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole, MA 02543-1026, or online at http://www.nefsc.noaa.gov/nefsc/publications/. Note that the documentation may be easiest to obtain at http://www.nefsc.noaa.gov/publications/crd/crd1114/index.html

Examples

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astrocalc4r(day=12,month=9,year=2000,hour=12,timezone=-5,lat=40.9,lon=-110)

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