Parallactic Angle and Solar Physical Ephemeris

In the solar eclipse simulator in Photo Ephemeris, you may optionally display data for the parallactic angle and the solar physical ephemeris. Tap the Sun icon near the top left to show it:

Parallactic Angle

The Parallactic Angle, q, represents the angle between the observer's zenith (i.e. the line from the center of the Earth through the top of your head and off to infinity and beyond), and the North Pole of a body, such as the Sun.

By convention it is negative until the body transits the southern meridian and thereafter, positive.

You can use this angle to estimate how the position of a sunspot, for example, will change over the course of a day's observation. Images of the Sun published by NASA and others, for example that shown on the home page of spaceweather.com are shown with solar north at the top.

It's not possible to show the appearance for every location on Earth and every moment of the day, so instead a "solar north up" image is shown:

Say we wanted to visualize how the position of sunspot group 3628 evolves over the course of the eclipse. We can estimate that by rotating the image by the parallactic angle, as shown here for the solar eclipse of April 8th in northern Mexico:

The intermediate pictures show the mid points between C1 and Max eclipse and Max eclipse and C4 respectively. With this in mind, we can gauge from the simulator when the sunspot group will be obscured by the Moon, and - even more usefully - when it is likely to be revealed once again after C3:

Armed with this information, you can plan to photograph or video the occultation of the sunspot group by the Moon.

Solar Physical Ephemeris

The angle P is the "position angle of the northern extremity of the axis of rotation, measured eastwards from the North Point of the solar disk" (Meeus).

E(P) together with P are explementary angles, that is, E(P) is equal to 360° minus P. It is included as a convenience for determining the angle by which to adjust the rotation of your camera.

If you are using an equatorial mount and wish to align your camera relative to the North Pole of the Sun, this angle (or the explementary angle) tells you what rotation you will need to apply.

An example use case: the corona of the Sun is predicted to reach its greatest extent in a particular direction, you may wish to align your camera's widest field of view with that point. You can read more on the topic over on the forum.