Comet Ephemerides with Geometry and Visibility

Info

by Steve Albers

Ephemeris Output● DEC, RA, Distance from Sun and Earth

● Magnitude, Tail Length, Tail orientation

● Phase Angle, Elongation

● Rise/Set Times, Time/Altitude for best

viewing

● Visibility compared to naked eye, binocular

thresholds

○ expressed as “effective” or corrected

magnitude

● Output computed daily or more frequently

Visibility Computation

● Effective Magnitude Adjusted from Actual

Magnitude

○ compare to naked eye 6.0 threshold

● Effective Magnitude Adjustments

○ extinction

○ sky brightness (moonlight, twilight, daylight

at best observation time)

Sky Brightness Computation

● Nighttime

○ moderate light pollution assumed,

increasing near horizon

○ increased by scattering/glare from the moon

● Twilight

○ empirical relationship of limiting magnitude

and comet/sun altitude difference

● Daytime

○ scattering/glare based on elongation from

sun, comet altitude and solar altitude

Internet Demo?

http://laps.noaa.gov/cgi/albers.homepage.cgi

(optional)

Visibility Examples● PANSTAARS reaching (barely) naked eye

magnitude limit in March

● Ikeya-Seki visible naked eye in Japan within

hours of perihelion, otherwise head was likely

invisible naked-eye (despite impressive tail)

● ISON orbit similar to Ikeya-Seki, though will be

dimmer and may never reach naked-eye

brightness

● Hale-Bopp longest most visible comet in last

50 years

● McNaught (2007) visible naked eye from

Longmont area in bright twilight, and in

binoculars during the daylight

Sky Brightness Computation

● It’s fine to calculate sky brightness and

limiting magnitude at individual points in the

sky for an ephemeris, however…

● Wouldn’t it be nice to show an image of sky

brightness over the entire sky??

Simulated All-Sky Images Compared with the LAS All-Sky

Camera

by Steve Albers, Vern Raben, and the NOAA LAPS Group

Simulation Ingredients

● 3-D Gridded Cloud Analyses (or

forecasts)

○ Cloud liquid, ice, rain, snow,hail

○ NOAA’s LAPS model (developed at

ESRL)

● Locations of Sun, Moon, Planets, Stars

● Specification of Aerosols (haze)

● Specification of Light Pollution

● Specify Vantage Point

○ Latitude, Longitude, Elevation

LAPS Cloud

analysis

METARMETAR

METAR

OAR/ESRL/GSD/Forecast Applications Branch 10

First Guess

Visualization Technique

● Illumination of clouds, air, and terrain are pre-

computed

● Sky brightness based on sun, moon, planets,

stars

● Ray Tracing from Vantage Point to each sky

location

● Scattering by Intervening Clouds, Aerosols, Air

● Terrain shown where its along the line of sight

● Physically and Empirically based for best

efficiency

Image Navigation● Overall correction based on optical axis centering,

spherical rotation, and radial lens “distortion”

• Need to rotate around Lambda Draconis?• Except that near horizon offsets are just in azimuth

(zenith rotation)

Cloud Illumination Example

Cloud Illumination (and scattering)

Nighttime Clouds (and stars)

Background Sky Brightness

● Source can be sun or moon

● Rayleigh Scattering by Air Molecules (blue sky)

○ Minimum brightness 90 degrees from light

source

○ Blue-Green sky color near horizon far from

sun

● Mie Scattering by Aerosols (haze)

○ Brighter near the light source (aureole)

● Added sky brightness from planets, stars, light

pollution, airglow

Daylight Clear Sky

Nighttime Comparison

Clear Air Illumination● Cloud shadows in clear air can show

crepuscular rays

● Brightness and color changes shown during

twilight

○ 3-D orientation of Earth’s shadow considered

○ Secondary scattering needed to reduce

contrast in Earth’s shadow that appears

opposite the sun

Twilight Comparisons

Twilight Comparisons

Terrain Illumination

● Topography data allows showing mountains

near the horizon

● Terrain Albedo (e.g. a dark forest)

● Adjusted by cloud shadows

● Show snow cover (future enhancement)

● Terrain can be obscured by intervening clouds,

haze, or clear air (very long distances)

Main Ray-Tracing Step● Trace from viewer into sky at ~1x1 degree grid

● Ray path travels through clear air, aerosols,

clouds, and may hit terrain

● First estimate is clear sky value (background

sky)

● Scattered by clouds (can show up either bright

or dark)

○ depends on optical depth of cloud and

elongation from sun, as well as pre-computed

cloud illumination

● Cloud/Aerosol scattering can obscure distant

terrain

More on Cloud/Precip Scattering● Mie scattering phase function means thin clouds

are brighter near the sun (silver lining), cloud

corona

● Thick clouds are the opposite, being lit up better

when opposite the sun

● Rayleigh scattering by clear air can redden

distant clouds

● Future enhancement would be to add rainbows &

halos

○ (with clouds/precip at specific elongation

angles)

Final Display

● Cylindrical grid (panoramic view) can be

calculated at either 1x1 or 0.5x0.5 degree

spacing

● Currently just shows at and above the horizon

○ future enhancement to show below the

horizon

● Convert to polar grid (shown here)

○ good for overhead views, and for camera

comparison

Cylindrical Panoramic View (½ degree resolution)

Example Animation #1

Example Animation #2

Internet Demo of All-sky Web page?

(optional)

http://laps.noaa.gov/allsky/allsky.cgi

What’s next?

The sky is the limit!