Sketching a Partial Lunar Eclipse with a 70-90 mm Refractor

Timing the eclipse

Eclipse dates decide the season

A lunar eclipse can only happen when the Moon is full, so the season is set by the eclipse date rather than by your choice. Check the date and the local times of the eclipse phases in an eclipse table, an almanac or a planetarium program set to your location. Plan for the eclipse you have, and do not assume a particular month will offer one.

Full Moon's position in the sky changes with the season. In the Northern Hemisphere, a full Moon in the cold half of the year climbs high, which can make the shadow easier to draw. In the Southern Hemisphere, the same is true during the local winter. A summer full Moon stays lower for much of the night, so expect more haze and turbulence near the horizon.

Pick the part of the night the Moon is up

The eclipse is visible from any place on Earth's night side while the Moon is above the horizon. Your job is to find out which phases fall after local dusk and before local dawn. If the Moon sets or rises partway through the eclipse, you will see only part of the sequence. Note the moonrise and moonset times for your location, then mark the eclipse phases that fall within that window.

If the greatest part of the eclipse falls during the hours when the Moon is up, plan to start your sketches before the first contact time, so that you have a reference drawing of the unshadowed Moon. Begin by drawing the Moon as it looks just before the shadow touches it.

Altitude controls the view

Altitude is the angle of the Moon above the horizon. Low altitude means more air to look through, more dispersion, and more shimmer. Near the horizon, the Moon can appear distorted and the shadow edge hard to pin down. Higher altitude gives steadier views and more reliable sketches.

Your latitude changes the Moon's path across the sky. The same eclipse will therefore peak at different altitudes from different places. For a site near 35° N, 50° N or 35° S, the Moon reaches a different maximum height at the same time of night. Before the event, use a planetarium program or ephemeris to find the Moon's altitude at your exact location for each eclipse phase. Do not rely on a single number from a general source, because the altitude depends on the date and your coordinates.

As a rule, wait until the Moon is clearly above the rooftops and trees around your site before you start. If the eclipse begins while the Moon is still low, take a short early sketch to record the first contact, then continue when the Moon is higher.

Twilight and dark adaptation

A full Moon is bright, so twilight matters less for seeing the Moon itself than it does for seeing the penumbra and the faint color within the shadow. Start your sketches when the sky is fully dark, which means after astronomical twilight has ended at your site. The Moon will be bright before that, but the sky brightness around it will still reduce contrast in the outer parts of the shadow.

If you need to read a star chart, a timing sheet or a notebook between sketches, use a dim red light. Red light preserves dark adaptation in your eyes, so you can see the shadow edge clearly when you return to the eyepiece.

Moon phase is part of the event

The eclipse requires a full Moon, so the Moon phase is already fixed. Its brightness is what makes lunar eclipses easy to observe from a suburban site, but it also creates glare. Use a low-power eyepiece to keep the whole disk in the field, and use a light shade or hood to reduce stray light from the eyepiece. Avoid working at the same time as a nearby streetlight shining into your telescope, since it can cause reflections across the field.

Suburban conditions and what they change

A suburban sky typically reaches a naked-eye limit of about magnitude 5. The Moon itself is far brighter than that limit, so you can see its surface and the shadow without difficulty. Light pollution does not prevent the eclipse from being seen. It does reduce contrast in the faint outer part of the penumbra, where the shadow fades gradually, and it can wash out color within the shadow.

For this reason, judge the shadow edge by its sharpness and position rather than by its darkness. The umbra's edge is usually the easiest feature to record, while the penumbral fade is harder. If you can see the penumbra clearly from your site, sketch it as a separate zone. If you cannot, record only the umbral boundary and note that the penumbra was not clearly visible.

Local light also affects how easily you can see the color of the shadow. The umbra may look reddish or copper-colored on a clear night, but the tone depends on the amount of dust and cloud in Earth's atmosphere along the shadow's path. Record what you see, and do not assume a particular shade for every eclipse.

Sketching method

Equipment

A 70-90 mm refractor is a suitable instrument for this target. Use a low-power eyepiece to frame the whole Moon, which typically means a magnification in the range of about 30x to 50x for that aperture. For craters and the shadow edge, switch to a higher magnification, usually about 80x to 120x, if the air is steady. These are typical ranges for guidance, not fixed settings.

You will also need:

• A sketchbook or sheets of plain paper, with a circle template or a drawing compass to draw the Moon's limb
• A soft pencil, a sharp pencil and an eraser
• A dim red light for reading notes and checking times
• A watch or phone clock, set to a reliable time source
• A timing sheet with columns for time, seeing, transparency and notes

Drawing the Moon

Start with an outline of the lunar disk. Use a template or a compass to get the circle right, then mark the shadow's position at each sketch. Draw the outline lightly, because you will add detail on top of it.

Sketch the features that are easy to identify: the large dark maria, the bright craters with clear rims, and the crater groups that are in or near the shadow's path. Do not try to draw every small crater. Work on the areas that will show the shadow's progress.

Record the shadow edge as a curve, not a straight line. The edge is roughly circular, so its curve matches the Moon's limb. Note which side of the disk is in shadow and where the edge is at each time.

Timing and intervals

Make a sketch at regular intervals. Ten to fifteen minutes between sketches is a reasonable starting point during rapid changes, and twenty to thirty minutes is often enough when the shadow is moving slowly. Adjust the interval to your speed of drawing and the seeing conditions. Each sketch should carry the time, which you read from a reliable clock at the moment you begin the sketch.

Write the seeing and transparency next to each drawing. Seeing describes how steady the image is, and transparency describes how clear the sky is. Use short notes such as "steady, clear" or "shimmering, hazy". These notes help you explain any differences between drawings later.

Recording the shadow's progress

Mark the shadow's edge with a thin line and label the time. As the shadow moves, draw the edge in its new position without erasing the earlier lines. Overlaying positions this way shows the shadow's path across the Moon.

Craters are useful markers. The shadow's edge does not hit all craters at the same moment. A crater can disappear into the shadow while its neighbor is still in sunlight. Draw each crater's last visible moment, then note the time it vanished. When the shadow leaves the Moon, the craters on the exit side will emerge from the dark area in the reverse order.

What to expect in the eyepiece

With a 70-90 mm refractor, you should expect to see craters disappear into and emerge from the shadow. The shadow's edge will look soft in some places and sharper in others, depending on the seeing and the local sky. Some craters will remain visible within the shadow for a short time before they fade, while others may vanish quickly.

Do not expect the shadow to look like a black disk. A partial eclipse produces an uneven shadow, and the Moon's surface inside it is still faintly visible in many cases. The most reliable feature for a sketch is the sharp outer boundary of the umbra, not the total darkness within it.

The eclipse is safe to view with the naked eye and with any instrument. You do not need a solar filter, because the Sun is not in the view.