Determination of 1 Shawwal: Bosscha Observatory ITB Provides Astronomical Information
The Bosscha Observatory at ITB in Lembang, West Bandung Regency, has communicated astronomical information regarding the position of the crescent moon on Thursday, 19 March, coinciding with 29 Ramadan 1447 Hijri. Based on observations, the crescent moon’s position will be very close to the sun in the western sky at sunset. As an educational and research institution in the field of astronomy, Bosscha Observatory conducts observation of the young crescent moon almost every month.
Each year, Bosscha Observatory serves as one of the principal references for determining the beginning of Islamic months, including Shawwal, for Indonesia’s Ministry of Religious Affairs and the general public. Based on calculations by Bosscha Observatory, astronomical data for Thursday, 19 March 2026, which corresponds to 29 Ramadan 1447 Hijri, indicates that the moon’s position will be extremely close to the sun in the western sky at sunset. The geometric parameters of the moon show that geocentric elongation (the angular distance of the moon from the sun as viewed from the centre of the Earth) across Indonesian territory will range from approximately 4.6° to 6.2°, from eastern to western regions.
Yatny Yulianty, a researcher at Bosscha Observatory, explained that topocentric elongation (the angular distance of the moon from the sun from the perspective of an observer on Earth’s surface) will range from approximately 4.0° to 5.5°. The moon’s altitude at sunset will also be relatively low. The altitude map of the crescent moon indicates that the hilal’s elevation across Indonesia will range from 0° to 3° above the horizon in western Indonesian regions. This condition indicates that the moon will be positioned near the sun in the western sky and at a low altitude above the horizon.
“Astronomically, this condition demonstrates that the hilal is at a challenging threshold for observation. The success of observation will be significantly influenced by atmospheric conditions, sky transparency, and the experience and observation methods employed,” she explained.
Yatny noted that to document the visibility conditions of the crescent moon, astronomers at Bosscha Observatory will conduct observations using telescopes and imaging instruments at two locations: Bosscha Observatory in Lembang and Lhok Nga Observatory in Aceh, supported by the Ministry of Religious Affairs. Lhok Nga Observatory was selected because the moon’s positional parameters in the Aceh region fall around the boundary of the currently applied crescent visibility criteria, making observations at that location important for verifying the boundary conditions through direct observation.
“This observation activity also forms part of Bosscha Observatory’s long-term research on crescent visibility, aimed at enriching the observational database of the young crescent moon across Indonesian territory,” said Yatny, who is also the Coordinator of Public Activities for the Education and Public Outreach Division at Bosscha Observatory.
According to Yatny, the determination of the beginning of Islamic months, including Shawwal, remains the authority of the government through the Ministry of Religious Affairs in the isbat session on 19 March 2026. However, Bosscha Observatory serves to communicate the results of calculations, observations, and crescent moon research as scientific input that can inform consideration in the decision-making process.