A simple example of a conjunction
is the New Moon, which occurs every month and is nothing more than a
conjunction of the Sun and Moon.
In astronomy, conjunctions are
defined in two ways: Conjunction in Right Ascension and Conjunction
in Ecliptic Longitude.
Conjunction in Right Ascension
First, let us look at a conjunction
in Right Ascension (RA). For this type of conjunction, the celestial objects
must have the same Right Ascension, meaning they are equally distant from the
First Point of Aries when measured along the celestial equator.
For the Venus–Jupiter conjunction
on 9 June, both planets have the same Right Ascension at 6:00 PM IST:
As shown in the chart, the planets
align parallel to the Right Ascension lines. Another way of stating this is
that they are aligned perpendicular to a declination line. This configuration
is known as a conjunction in Right Ascension.
Conjunction in Ecliptic
Longitude
Next, let us look at a conjunction
in Ecliptic Longitude.
Ecliptic Longitude is the angle
measured eastward along the ecliptic from the First Point of Aries. At first
glance, this sounds similar to Right Ascension because both measurements begin
at the First Point of Aries and increase eastward. However, the key difference
lies in the reference plane used.
Right Ascension is
measured along the celestial equator and is expressed in hours, minutes, and
seconds, where 15 degrees equals 1 hour.
Ecliptic Longitude, on the
other hand, is measured in degrees along the ecliptic, which is the apparent
path of the Sun across the sky.
Since the ecliptic is tilted by
approximately 23.44° with respect to the celestial equator, the two coordinate
systems are not identical. As a result, a conjunction in Right Ascension and a
conjunction in Ecliptic Longitude generally occur at different times.
In the chart below, the grid lines
represent the Right Ascension and declination coordinate system, while the
yellow line represents the ecliptic. Compare this with the ecliptic longitude
chart, where the division lines are drawn perpendicular to the ecliptic.
By examining the orientation of these lines, we can clearly see why the conjunction in Right Ascension and the conjunction in Ecliptic Longitude occur at different times.
Let us examine both conjunctions in
more detail using star charts with coordinate lines.
Conjunction in Right Ascension
The chart shows that, in the
Equatorial Coordinate System, the planets are aligned parallel to the Right
Ascension lines. This indicates that they are equidistant from the First Point
of Aries when measured along the celestial equator.
If we now switch the same chart to
the Ecliptic Coordinate System, it becomes visually obvious that the planets
are not equidistant from the First Point of Aries when measured along the
ecliptic.
Conjunction in Ecliptic
Longitude
The chart below shows that the
planets are now aligned parallel to the coordinate division lines that are
perpendicular to the ecliptic. This indicates that the planets are equidistant
from the First Point of Aries when measured along the ecliptic.
In other words, the planets had to
move a little farther after the Right Ascension conjunction before achieving
alignment in Ecliptic Longitude.
Conclusion
When you observe the Venus–Jupiter
conjunction on the evening of 9 June, I hope you will remember that there is
more than one way to define a conjunction. Whether it is a conjunction in Right
Ascension or a conjunction in Ecliptic Longitude depends entirely on the
coordinate system being used.
Understanding the difference
between these two types of conjunctions provides a deeper appreciation of how
astronomers measure positions in the sky and why the same celestial event can
have slightly different timings depending on the coordinate system chosen.
SKY FOR THE WEEK
The concepts presented in this article were discussed during the Sky for the Week session held on 7 June 2026 at the Association of Bangalore Amateur Astronomers (ABAA). In these weekly Sunday sessions, members and visitors explore upcoming celestial events, learn how to observe them, and discuss the astronomical principles behind what we see in the sky.
From planetary conjunctions and lunar events to eclipses, meteor showers, and seasonal sky highlights, the sessions aim to connect celestial observations with the science that explains them. Understanding not only what happens in the sky, but also why it happens, helps observers develop a deeper appreciation of astronomy.
The Sky for the Week sessions are conducted every Sunday and are open to anyone interested in exploring the wonders of the night sky.
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