Moonchaser Usage Notes


This page has been mostly unmodified during the 21st century. ...so it's insanely outdated.


Chasing the Moonrise: Step-by-Step

Known Problems

Basic Photographic Issues

Photographic Equipment

Choosing a Film

Moonchaser Home Page

Chasing the Moonrise: Step-by-step

Moonchaser is a tool that helps you be in the right spot at the right time to photograph the moon rising behind a foreground object of your choice. The photographs on this site were taken with the aid of previous versions of Moonchaser.

Gather Your Equipment

The minimum equipment you need consists of a camera and a topographic map of the area that includes your target object and the area to the west of that object. Additional recommended equipment includes a telescope or long focal length (telephoto) lens, and a sturdy tripod. See the sections below on photo equipment film selection.

While most of the photographs on this site were taken before inexpensive handheld global positioning system (GPS) receivers were available, the use of one greatly simplifies finding the perfect spot from which to shoot. If you do use a GPS check your setup menu and be sure to use the NAD 1927 (Conus) datum. Many GPS units default to using the WGS 84 datum instead, and that causes small errors with most USGS topographic maps.

Identify & Locate the Target Object

Choose something to be in your picture in front of the rising moon. For starters, pick something easy. A building, like Lick Observatory, that sits on the top of a high mountain is a good choice. You want something that can be seen from many locations to the West. As the moon's position shifts from night to night and from season to season you will have to move around to be in the proper position to catch your photo. Find the object on your topographic map and write down its position in UTM coordinates. Most USGS topos have UTM grid ticks every 1000 meters, often in blue. You will also want to write down the UTM zone. This is probably shown in the little paragraph in the lower left hand corner of the map that talks about UTM grid ticks and their spacing. The UTM coordinates for Moonchaser are in meters, so the "Easting" numbers are 6 digits and the "Northing" numbers are 7 digits. Since the tick marks on the map occur every 1000 meters, the numbers shown on the map are usually 3 and 4 digits respectively. Add three zeros to locations that lie on grid lines and estimate the last three digits for the rest.

If you're confused by the Universal Transverse Mercator system, the National Park Service, the USGS, and MapTools, Inc. all have information that will help.

Don't forget to determine the elevation of the target. If you have chosen a tall object such as a tower or a skyscraper, you will want to add all or part of the height of the object to the elevation you read from the map.

Wait for the Full Moon

It's not just that the full moon is more photogenic. The full moon rises just about sunset. That's perfect. If the sun is too high in the sky when the moon rises, there will be insufficient contrast between the moon and the sky to get a good picture. (If you try to find a moonrise more than about 45 minutes before sunset, you may not even be able to see the moon until it is several degrees above the horizon). If the sun has already set, you won't have enough light on your target object to get anything but a silhouette

Guess at the Camera Position

Consult your topographic map again and pick a spot to start your chase. Don't take too much time choosing the first spot, you'll change it when you start looking at the data. We usually just enter the coordinates of the back yard, although in our case the back yard sometimes works for the full moon rises nearest the Vernal and Autumnal Equinoxes. Enter the elevation of the point where you start.

Enter Your Data Into the Moonchaser Calculator

Take the data you have written down and enter it into the Moonchaser Calculator data entry form. Take care entering the date. All times and dates used by Moonchaser are specified in Coordinated Universal Time (UTC). UTC is what used to be called Greenwich Mean Time (GMT). For most locations in the U.S., it will be 4 to 8 hours ahead of local time. This means that in the summer on the West coast, it isn't unusual to have moonrises happening just about midnight UTC. Be careful you get the right date, or you'll miss your target by several moon diameters.

When you have entered the UTC date, the UTM coordinates and elevation of both your camera and target locations, and the UTM zone, you're ready to click the "Calculate" button and see what happens.

Understanding Moonchaser Output

If you get all of your data entered correctly, the Moonchaser Calculator will reward you with a prediction of the moonrise. You will see three sets of rise and set times for the sun, and two for the moon. The time of the sunset is important in guaging how bright the sky will be when the moon rises.

The moon rise time at the horizon, and at the elevation of your target will be printed, along with the phase, expressed as a percentage of full, and the apparent diameter of the moon in degrees. The moon rise time at the horizon should agree with the published moonrise times in your local paper (after correction to the local time zone). The number after the rise and set time is the bearing to the sun or moon measured from true North.

Below the table of rise and set times are several lines that describe how close to your target the moon will rise. Since the moon is about one half a degree in diameter, in order to have the image of the moon behind your target object, you must have an error of one quarter of a degree or less. Keep in mind that when the moon rises it does not move straight up, but follows a path from left to right. Therefore you may want to "miss" slightly to the left (North) and the moon will pass diagonally from lower left to upper right behing your target.

Moonchaser also makes a suggestion about where to move to improve your positioning for the shot. Moonchaser always suggests moving either due North or due South, but you can adjust your position East and West as well. (For moonrises in the Northern Hemisphere after the Vernal Equinox and before the Atumnal Equinox moving East has the same effect as moving North. For moonrises in the Northern Hemisphere after the Atumnal Equinox and before next Vernal Equinox, moving East has the same effect as moving South... but you probably already guessed that didn't you?)

As an aid to planning your move, Moonchaser prints two "UTM Intercepts". These are two sets of UTM coordinates that define a line from the apparent position of the moon through your target point and through the 1000 meter UTM grid lines on either side of your camera position. By pencilling this line in on your topo map you can see a set of possible camera positions. Keep in mind that this line is only valid for areas that are the same elevation as the camera position you entered.

Iterate Until You Find Your Spot

At the bottom of the page there are data entry fields to enable you to calculate a revised camera position. Moonchaser fills these in with the position of one of the UTM Intercept points, but you can change them to any UTM coordinates you choose. Be sure to look up the elevation of your new camera position and enter that before your click the "Recalculate" button.

Continue this virtual wandering around your topo map until you find a spot where you're happy with the times and angles. Be sure to consider what's located between your camera position and your target position before you decide you're finished. A tall building or a nearby hill can obscure the view.

There are also other considerations to keep in mind. Remember that you are going to be setting up a camera with a telephoto lens, and maybe even a telescope, and that you will be pointing it at a rather low angle with respect to the horizon. While you know that you are going to be pointing at the moon when it rises, it can look to the neighbors (and the local constabulary) like you are pointing your very long lens into the windows of that apartment building located just below your target.

When the police arrive... and they quite often will, invite them to peek through the viewfinder as you explain what it is you are really doing. A little P/R goes a long way.

But there are limits. We have learned first-hand that setting up cameras and telescopes on the sidewalk in front of the "Federal Systems Division" of a Silicon Valley high-tech company results in rather prompt and intense discussions with corporate security folk. As a group, these people often lack the imagination and esthetic sensibilities to fully appreciate the sport of chasing the moonrise. They appear to have a generally underdeveloped sense of humor as well.

Pack-Up and Go

Once you have identified the perfect spot (at least on the map) it's time to pack up all your gear and head out into the field. As you might have figured out by now, it would be a good idea to leave early and have lots of time to set-up. Advance planning pays off. If you are not very familiar with the area near your intended camera location, it's a good idea to do all the calculations a couple of days in advance so you can visit the spot and make sure there's not a tree or a building (or a 5000 foot mountain) blocking your view of the target.

Share Your Results

Be patient, and expect your first couple of attempts to fail. It will take a little practice before you get the feel of how to judge the best combination of sunset and moonise times to get the perfect photograph. Much depends upon the local terrain. If you have a range of mountains to your West, the sun may effectively "set" before the computed times. Report any problems you have with Moonchaser, and if you get some photos you're proud of, put them up on the WWWeb and let us know so we can share the URL with other Moonchasers.

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Known & Suspected Problems

The Elusive 2 Minute Error

Versions 3 through 6 of Moonchaser have suffered from an error of about two minutes in the calculation of moon rise times. Bearings appear correct, but the moon always rises about two minutes later than the program predicts. We have been looking for the cause of this for years without success.

Input Form Tab Order

When used on versions of the Netscape Navigator browser before version 6, the primary data input form for Moonchaser has an inconvenient tab order. When entering the camera and target positions using the TAB key to move between fields alternates between the Camera Location fields and the Target Location fields. This does not appear to be a problem on Netscape Version 6 nor on recent versions of MS Internet Explorer.

Iterative Position form Position Input Boxes

Moonchaser calculates a suggested set of UTM coordinates and uses them as the default values in data input fields at the bottom of the Moonchaser results page. Clicking on the button at the bottom of the page causes recalculation of the results using this new data as the camera position. When viewed with some versions of the Netscape Navigator browser, the default values are not properly centered in the input box. This problem does not appear to occur when using the versions of the Microsoft Internet Explorer browser that have been tested to date.

UTM Boundary Effects

The Universal Transverse Mercator projection system was worked-out by the Army for the purpose of lobbing artillery shells about the countryside and getting most of them to hit their intended target. (And presumably to have few, if any, hit unintended targets). It provides a convenient rectilinear notation for making such calculations on the surface of an ellipsoidal planet such as Earth. To do so, it divides the surface into zones, and provides a coordinate space within each zone. Moonchaser has been used primarily in the San Francisco Bay area, and consequently its accuracy has not been tested near zone edges. In the author's opinion, it is unlikely to provide satisfactory performance near the zone edges, particularly if the camera and target positions are in different zones. But then the author is neither a cartographer nor a mathematician, so maybe it will work OK after all. If you have any experience with Moonchaser near or across the edges of zones, please drop us an e-mail and let us know what happened. We'll share the information with everyone here on this site.

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Basic Photographic Issues

<to be written>   ...and now a dozen years later it isn't likely to be.

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Photographic Equipment

<to be written>   ...neither is this.

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Choosing a Film

Yes, this was written back when we all shot our photos on film.

For those under 20, film was long strips of acetate coated
with a gelatin emulsion of sensitized silver halide.

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