# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Working a lunar. was: lunar distance in Wikipedia**

**From:**Dave Walden

**Date:**2009 Jul 24, 09:16 -0700

Generating a Lunar Distance example (with some simplifications to make things easier/more understandable/not much less accurate. If this offends you, please stop reading now!) Go to: http://aa.usno.navy.mil/data/docs/celnavtable.php Pick an appropriate date, time, and location (i.e. the moon is up and situated reasonably). Enter your selections. For example: Date and time of observation: Year: 2009 Month: August Day: 1 Hour: 2 Minute: 47 Second: 45 UT Assumed position: Enter best-estimate sea level coordinates. Latitude: X north south 38 o 53 ' Longitude: east X west 77 o 01 ' You'll get a table of data. We will use Jupiter as our body. Just to get rolling and to make a point, we'll first calculate the true Lunar Distance (center to center). First we'll do it in celestial coordinates: True LD = asin (sin Jup Dec *sin Moon Dec + cos Jup dec * cos Moon Dec * cos (Moon GHA - Jup GHA) = 68 deg 20.9 min Second we'll do it in local alt-az coordinates using calculated altitudes, Hc: True LD = asin (sin Jup Alt *sin Moon Alt + cos Jup Alt * cos Moon Alt * cos (Moon Az - Jup Az) = 68 deg 22.0 min Whoa! They should be the same! (Note the parallelism in the equations. Same would be seen in the Great Circle Distance equation. All the same triangle.) Problem is that the USNO gives only 0.1 deg (6 min) resolution in Az. Fine for plotting and star ID; not good enough for LD calculation. Luckily, we have all we need to do our own calculation of Az. Moon Az = 180 + ATAN2(COS Moon LHA * SIN Lat - TAN Moon Dec * COS Lat, SIN Moon LHA ) (Careful with the order of Atan2 arguments, it varies! Above is MS EXCEL order.) ((so why atan2, not atan? Because it keeps things in the correct quadrant with no fuss!)) where, Moon LHA = Moon GHA + Long (-West) = 21.028 Moon Az = 200.270 deg Likewise, for Jupiter, Jup Az = 129.990 deg Using these values for Az in our alt-az LD equation, we get: True LD = 68 deg 20.9 min Well, that's a relief. It agrees, as it must! (The bodies are in the same places with the same separation whatever coordinate system we might choose. Since separation is a relative quantity, it must be the same.) Now that we know how to calculate LD in local alt-az coordinates, we are ready to calculate the observed LD. (Which is what we need to generate an example.) We just use the above equation replacing calculated altitudes, Hc with observed altitudes Ho. (Assume IC=0, Dip=0, or already corrected out.) In the USNO notation, Ho = Hc - Sum of Altitude Corrections. BUT, since we observe separation from near or far limb of the Moon, not the Lower Limb, we don't use the SD part of Moon correction. So what we'll calculate is separation to the center, adding or subtracting the SD for far or near limb respectively. Moon Ho = 20 deg 48.6 min (to LL, to use with Frank) or 21 deg 3.5 min (to center, to use in Observed LD calculation) Jup Ho = 18 deg 2.0 min (to LL, since USNO includes a Jupiter SD correction. Some, Frank for instance, assume we have observed center. Difference is .4) so, use 18 deg 2.4 min (to center, for Frank observed altitude AND Observed LD calculation) ((although body altitude has small effect)) Observed LD = Moon SD + asin (sin Jup Ho *sin Moon Ho + cos Jup Ho * cos Moon Ho * cos (Moon Az - Jup Az) = 68 deg 44.8 min That's it. Done! It you paste the below URL, you'll get Frank's result for our example. Quite close. http://www.historicalatlas.com/lunars/lunars_pre.asp?LatDeg=38&LatMin=53&LatName=N&LonDeg=77&LonMin=1&LonName=W&IC=&temp=50&press=29.80&hteye=0&BodySel=Jupiter&BodyAltDeg=18&BodyAltMin=2.4&BodyLimb=LL&MoonAltDeg=20&MoonAltMin=48.6&MoonLimb=LL&GMTmonth=August&GMTday=1&GMTyear=2009&GMTHrs=2&GMTMin=47&GMTsec=45&gmtsel=Greenwich+Mean+Time&LunarDeg=68&LunarMin=44.8&LunarName=Far&obcheck=on&flatcheck=on (We have ignored flattening and oblatness. In the case of the Sun, the Moon SD discussion applies.) In the attached spreadsheet, enter the green from USNO (or elsewhere) and use red. --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---