Photometric variability of Uranus and Neptune, 1950–2004
Introduction
This paper combines three distinct sets of annual observations, all obtained at Lowell Observatory, that together provide over a half century of photometric coverage of Uranus and Neptune with only a relatively short hiatus from 1966 to 1972. Broadband B and V measurements on the UBV photometric system cover the interval from 1950–1966. Intermediate-band b and y filters of the Strömgren photometric system extend the series from 1972–2004. The B, V data come mainly from two Lowell Observatory Bulletins (Serkowski, 1961, hereafter S61; Jerzykiewicz and Serkowski, 1966, JS66).
New information in this paper, including our basic interpretation of the lightcurves, builds on previously published material (Lockwood and Thompson, 1999, hereafter LT99; and 2002, LT02), adding eight seasons of additional b, y magnitudes for Uranus and four for Neptune plus Lowell V data not previously included in the planetary variability record. A companion paper by Hammel and Lockwood (2005) presents additional interpretations of the planetary lightcurves.
Section snippets
Solar and planetary variability at Lowell Observatory
The motivation for this program a half-century ago was the desire, on behalf of weather and climate scientists, to assess possible solar variability by monitoring sunlight reflected from planets. Postwar improvements in photomultiplier tubes made this effort technically feasible, offering at least the possibility of obtaining definitive evidence for solar variations. We now know from spacecraft measurements that the Sun's variations are very small, less than 0.1% over the 11-year sunspot cycle (
Observations
As in our earlier papers (LT99 and LT02), we combine data from S61 and JS66 with subsequent b, y photometry, but now include V photometry from 1954–1966. From the beginning, the solar variations program focused on differential B measurements, mainly to speed up the observational cadence. A few separate planetary B, V observations were included in the much broader program of comparison star measurements to obtain seasonal B–V color indices. These, never before published outside the Lowell
Magnitude transformations
To make conjoined lightcurves we must accurately transform and , applying constant offset values determined from observations made in all four filters in 1973 and 1975. The offsets differ slightly for each planet since their spectra are different, and may change slightly over time if the planetary colors change significantly (possibly important for Uranus but not for Neptune). A second approach, subject to considerable uncertainty and therefore used here only for confirmation, involves
Lightcurve of Uranus
Uranus exhibits a sinusoidal seasonal 0.025 mag variation caused by its changing aspect viewed from the Earth. This “geometrical lightcurve” variation is , where Δm is the variation in magnitude units, i is the 97.9° inclination to the ecliptic, ɛ is the 0.023 oblateness, and θ is the sub-Earth latitude on the planetary disk. Fig. 2 shows the observed variability of Uranus with the geometric variation indicated by a dotted line. The B magnitude series that began in 1950
Conclusion
We have assembled a time series of photometric measurements of Uranus and Neptune covering more than a half-century, a substantial portion of a full orbit around the Sun for each planet. These provide data for the models by Hammel and Lockwood (2005) and indicate that the seasonal Neptune model proposed by Sromovsky et al. (2003) lacks an additional (unknown) major component. A seasonal model for Uranus that incorporates pole-to-pole latitudinal contrasts using the latest Hubble Space Telescope
Acknowledgements
We thank long-time colleague Heidi Hammel and the two referees for many suggestions, Lawrence Wasserman for ephemeris calculations, and Don Thompson for sharing observation and data reduction duties from 1976 until 1997. We acknowledge the critical role played by the late John S. Hall in keeping this program alive during his tenure as Director of Lowell Observatory from 1958 to 1975. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. Currently it is
References (48)
- et al.
Atmospheric structure of Neptune in 1994, 1995, and 1996 HST imaging at multiple wavelengths
Icarus
(1997) Spectrophotometry of the jovian planets and Titan at 300- to 1000-nm wavelength: The methane spectrum
Icarus
(1994)Methane, ammonia, and temperature measurements of the jovian planets and Titan from CCD-spectrophotometry
Icarus
(1998)Uranus' apparent seasonal variability in 25 HST filters
Icarus
(2001)Secular brightness variations of Titan, Uranus, and Neptune 1972–1976
Icarus
(1977)Analysis of photometric variations of Uranus and Neptune since 1953
Icarus
(1978)- et al.
Photometric variability of Uranus, 1972–1996
Icarus
(1999) - et al.
Photometric variability of Neptune, 1972–2004
Icarus
(2002) - et al.
Seasonal change on Titan observed with the Hubble Space Telescope WFPC-2
Icarus
(1999) - et al.
Titan's North–South asymmetry from HST and Voyager imaging: Comparison with grounbased photometry and models
Icarus
(1997)
Neptune's atmospheric circulation and cloud morphology: Changes revealed by 1998 HST imaging
Icarus
The nature of Neptune's increasing brightness: Evidence for a seasonal response
Icarus
UBVRI passbands
Publ. Astron. Soc. Pac.
Four-color and Hβ photometry for the brighter A0 stars
Astron. Astrophys. Suppl.
Keynote address: An historical review of solar variability, weather, and climate
Solar irradiance variation
Photoelectric magnitudes and color of Uranus
Astron. J.
Hubble Space Telescope imaging of Neptune's cloud structure in 1994
Science
Photoelectric reductions
A search for solar variations
Astrophys. J.
Photometry and colorimetry of planets and satellites
A library of stellar spectra
Astrophys. J. Suppl.
Cited by (45)
Spatial structure in Neptune's 7.90-μm stratospheric CH<inf>4</inf> emission, as measured by VLT-VISIR
2020, IcarusCitation Excerpt :Previous ultraviolet to near-infrared measurements of Neptune have demonstrated a wealth of variable phenomena in its upper troposphere. These include a series of dark oval features, the largest of which is denoted ‘the Great Dark Spot’ (Hammel et al., 1995; Smith et al., 1989; Sromovsky et al., 2001; Wong et al., 2018), latitudinal banding related to the distributions of haze and methane humidity (Karkoschka and Tomasko, 2011; Karkoschka, 2011) and cloud phenemona (Roddier et al., 1998, 2000; Max et al., 2003; Gibbard et al., 2002, 2003; Irwin et al., 2011) with variability on daily to annual timescales (Sromovsky et al., 1995, 2001; Lockwood and Jerzykiewicz, 2006; Hammel and Lockwood, 2007; Luszcz-Cook et al., 2010). VISIR’s 256 × 256 pixel array and pixel scale of 0.075” resulted in a total field-of-view of approximately 19 × 19” covering Neptune’s 2.34” diameter disk and background sky.
Computing apparent planetary magnitudes for The Astronomical Almanac
2018, Astronomy and Computing